I-NRLF 


or  PHOTOGRAPHY 

AND  PHOTOGRAPHIC  LENSES 


J.TRAILL  TAYLOR. 


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THE    OPTICS    OF    PHOTOGRAPHY 


AND 


PHOTOGRAPHIC  LENSES. 


GEOMETRICAL  OPTICS.     An  Ele- 

mentary  Treatise  upon  the  Theory  and  its  Practical 
Application  to  the  more  exact  Measurement  of  Optical 
Properties.  By  T.  H.  BLAKESLEY,  M.A.,  F.Phy.S. 
With  32  Diagrams,  zs.  (>d.  net. 

'  It  must  be  said  that  a  more  interesting  and  stimu- 
lating book  ....  is  seldom  likely  to  come  in  the  way 
of  the  student.  Mr.  Blakesley  has,  moreover,  effected  a 
notable  advance  in  geometrical  optical  theory.'— Nature. 

LENS  WORK  FOR  AMATEURS.     By 

H.  ORFORD.     With  numerous  Illustrations.     y. 

'  The  book  is  a  trustworthy  guide  to  the  manufacturer 
of  lenses,  suitable  alike  for  the  amateur  and  the  young 
workman . ' — Natu  re. 

'The  author  is  both  a  sound  practical  optician  and 
is  able  to  convey  his  knowledge  to  others  in  a  clear 
manner.' — British  Journal  of  Photography. 

MODERN   OPTICAL   INSTRUMENTS. 

By  the  same  Author.     With  88  Illustrations.     2s.  6d. 

CONTENTS  : — The  Eye  as  an  Optical  Instrument — 
Properties  and  Aberrations  of  Lenses — Aberrations  of  the 
Eye — Examination  of  the  Eye— The  Ophthalmoscope — 
Ophthalmoscopes  and  their  Uses— The  Morton  Ophthal- 
moscope— Various  Forms  of  Ophthalmoscopes—  Retino- 
scopy — Spectacles  and  their  Selection — Various  Forms 
oi  Spectacles  Illustrated  and  Described — Stereoscopic 
Projections  —  Anderton's  System  —  Principles  of  the 
Optical  Lantern — The  Stereoscope — The  Spectroscope. 

'  To  those  of  our  readers  who  wish  to  inquire  into  the 
elements  of  optical  instrument  construction  and  the 
principles  involved  therein,  we  can  cordially  recommend 
the  little  book.'—  Photographic  News. 


WHITTAKER  &  CO.,  LONDON,  E.C 


THE 


OPTICS  OF  PHOTOGRAPHY 


AND 


PHOTOGRAPHIC  LENSES 


BY 


J.  TRAILL  TAYLOR 


3fllUj3tration0, 


THIRD  EDITION  (REVISED). 

WITH    AN    ADDITIONAL    CHAPTER   ON    ANASTIGMATIC    LENSES    BY 

P.  F.  EVERITT. 


WHITTAKER   &   CO. 

2  WHITE  HART  STREET,  PATERNOSTER  SQUARE,  LONDON,    E.G., 
AND  66  FIFTH  AVENUE,  NEW  YORK. 

1904 


/  /  /  L//  rl 

Ifl/b  *  1 1  / 

/ 

QPTOMETflY 


753 


PREFACE 


THIS  little  treatise  is  not  theoretical  but  practical, 
and  it  is  not  intended  for  the  makers  but  the  users  of 
photographic  lenses. 

Some  of  it  is  already  familiar  to  readers  of  The 
British  Journal  of  Photography  and  its  Almanac,  and 
such  portions  are  reproduced  by  the  kind  permission 
of  the  Proprietor  ;  while  other  portions  are  collated 
from  my  contributions  to  the  Society  of  Arts,  The 
Photographic  Times,  the  Camera  Club,  and  various 
other  London  and  Provincial  Societies.  There  are, 
however,  several  chapters  written  expressly  for  this 
work,  while  in  every  case  the  other  matter  has  been 
entirely  revised  or  re -written  and  brought  up  to 
date. 

If  it  be  said  that  there  are  innumerable  lenses  in 
commerce  which  are  not  even  mentioned  by  name  in 
this  volume,  I  reply  that  each  maker  has  his  idio- 
syncrasy— he  may  vary  the  diameters,  foci,  and  curves 
of  his  productions,  and  select  special  trade  terms  by 


vi  PREFACE. 

which  to  distinguish  them,  but  I  have  preferred  in  all 
cases  to  associate  each  class  of  lens  with  the  name  of 
its  first  inventor,  and  believe  that  no  lens  in  use  at 
the  present  day  has  been  omitted. 

It  is  in  the  hope  that  the  work  will  prove  useful 
to  photographers  — both  professionals  and  amateurs — 
that  it  is  issued. 

J.  TRAILL  TAYLOR. 


PREFACE  TO  THIRD  EDITION. 

ADVANTAGE  has  been  taken  of  the  demand  for  a 
new  edition  to  thoroughly  revise  the  work  and  bring 
it  up  to  date  by  including  the  recent  period  of  anastig- 
matic  construction.  This  has  been  done  by  omitting 
the  short  chapter  on  Lenses  of  Jena  Glass,  written  by 
the  late  Mr.  J.  Traill  Taylor,  and  inserting  in  its  place 
one  on  Anastigmatic  Lenses,  specially  written  for  this 
book  by  Mr.  P.  F.  Everitt. 


CONTENTS. 


CHAP.  PAGE 

I.   WHAT  CONSTITUTES  PHOTOGRAPHIC  OPTICS  —  NATURE 

AND  PROPERTIES  OF  LIGHT I 

II.   PHOTOGRAPHIC  DEFINITION,  REAL  AND  IDEAL— FORMS 

OF  SINGLE  AND  ACHROMATIC  LENSES         ...  7 

III.   THE  CAUSE  OF  AN  INVERTED  IMAGE        ....  12 

IV.    SPHERICAL  ABERRATION 1 5 

V.   THE  NATURE  AND   FUNCTION  OF  THE  DIAPHRAGM  OR 

STOP 20 

VI.    PROPERTIES    OF    DEEP   MENISCUS    LENSES  —  COMPEN- 
SATING SINGLE  LENSES 27 

VII.   THE  OPTICAL  CENTRE  OF  SINGLE  LENSES        ...  34 

VIII.   THE  OPTICAL  OR  FOCAL  CENTRE  OF  A  COMBINATION     .  38 

IX.    SINGLE  ACHROMATIC  LENSES 42 

X.   DISTORTION  :    ITS  NATURE  AND  CURE  ....  49 

XI.   NON-DISTORTING  LENSES         .          .          .          .          .          .58 

XII.   WIDE-ANGLE  NON-DISTORTING  LENSES            ...  65 

XIII.  PORTRAIT  LENSES 73 

XIV.  RAPID  LANDSCAPE,  GROUP,  AND  COPYING  LENSES            .  80 
XV.    UNIVERSAL  LANDSCAPE  LENSES 87 

XVI,   FLARE  ANP  THE  FLARE  SPQT  ,,,,,,  93 


viii  CONTENTS. 

CHAP. 

XVII.   THE  EQUIVALENT  FOCUS IOO 

XVIII.   CONJUGATE    FOCI Io6 

XIX.    THE  PRINCIPLE  OF  CONJUGATE   FOCI   APPLIED   TO 

HAND  CAMERAS  AND  FOR  ENLARGEMENT    .  .112 

XX.    A    MECHANICAL    MEANS    OF    ESTIMATING     CONJU- 
GATE  FOCI Il6 

XXI.    DEPTH  OF  FOCUS 123 

XXII.    DIFFUSION  OF  FOCUS 128 

XXIII.    TESTING  LENSES 135 

XXIV.    THE  SHAPE  OF  THE  APERTURE  IN  THE  DIAPHRAGM  147 

XXV.    EQUALISING   THE    ILLUMINATION    OF    SUBJECTS-- 
SKIES AND  FOREGROUNDS     150 

XXVI.   ADJUSTING  DISSIMILAR  LENSES          .           .           .           .154 
XXVII.    THE  DETERIORATION  OF  LENSES  BY  LIGHT      .           .158 
XXVIII.    HOW    TO    ASCERTAIN    THE     ANGLE    OF    VIEW    IN- 
CLUDED  BY  ANY   LENS 165 

XXIX.   REFINED  FOCUSSING  BY  MEANS  OF  A  TELESCOPE     .       1 70 

XXX.   ANASTIGMATIC  LENSES     .          .          .  .          -  -177 

XXXI.   MOUNTS  AND  CELLS  ....  207 

XXXII.    LENS  GRINDING 214 

XXXIII.  OPTICAL  CONTACT— CEMENTING  LENSES  .  .224 

XXXIV.  SELECTION  OF  LENSES 230 

XXXV.    ON  THE  CURE  OF  EXISTING  DISTORTION  .  .      237 

XXXVI.   LANTERN  OPTICS— ENLARGING  AND  PROJECTING   .  241 

XXXVII.   PHOTO-TELESCOPIC  LENSES 2  =  8 

XXXVIII.    EXCEPTIONAL  RAPIDITY  WITH  HIGH  DEFINITION    .  260 

XXXIX.    MISCELLANEOUS         .           , 262 


THE  OPTICS  OF  PHOTOGRAPHY 

AND 

PHOTOGRAPHIC  LENSES 


CHAPTER  I. 

WHAT  CONSTITUTES  PHOTOGRAPHIC  OPTICS —NATURE 
AND  PROPERTIES  OF  LIGHT. 

PREVIOUS  to  speaking  of  the  lenses  employed  in 
photography,  or  the  principles  which  underlie  their 
construction,  it  will  be  necessary  to  explain  what  we 
mean  by  the  term,  the  Optics  of  Photography,  as  con- 
tradistinguished from  the  optics  of  any  other  science, 
such  as  those  which  involve  the  use  of  the  microscope 
or  telescope. 

The  chief  distinction  lies  in  this:  that  in  photographic 
optics,  not  only  must  those  rays  which  are  transmitted 
directly  through  the  lens,  or  the  axial  rays,  as  they  are 
designated,  be  brought  to  a  focus,  but  also  those  which 
pass  obliquely,  or  in  a  direction  other  than  axial.  The 
principal  lenses,  or  object-glass,  of  a  telescope  or  micro- 


•I  CHEMICAL  AND  VISUAL  FOCI. 

scope  will  not  give  a  sharp  image  if  removed  in  even  a 
slight  degree  from  perfect  squareness  of  position  in 
relation  to  the  line  of  light.  Hence,  the  sharpness  of 
image  produced  by  even  the  finest  telescope  object-glass 
is  confined  to  a  very  small  space  in  the  centre,  the  rest 
of  the  image  being  indistinct,  owing  to  the  inability  of 
an  objective  of  this  class  to  form  a  sharp  image  of  an 
object,  the  light  from  which  is  transmitted  obliquely. 

In  photographic  optics,  on  the  other  hand,  the  con- 
struction of  the  lens  must  be  such  as  not  only  to  give  a 
sharp  image  of  the  object  to  which  it  is  directed,  but 
also  of  those  which  lie  within  a  certain  extent  on  either 
side  of  the  centre.  In  proportion  as  a  lens  embraces 
objects  situated  at  a  considerable  distance  from  the 
point  to  which  it  is  directed,  so  does  such  lens  become 
entitled  to  the  designation  of  being  a  { wide-angle  '  lens. 

But,  further,  the  chief  end  of  any  optical  instrument, 
such  as  the  telescope  or  microscope,  formed  for  visual 
examination,  has  been  attained  when  it  is  made  to 
produce  an  image  that  is  sharp  when  examined  with  the 
eye.  But  with  a  photographic  lens  something  more  is 
required.  The  corrections  of  the  lens  must  recognise 
the  absolute  necessity  of  all  the  chemical  rays  being 
brought  to  a  focus  at  the  same  spot  as  the  visible  rays, 
so  that  not  only  will  the  image  appear  sharp  to  the  eye, 
but  it  will  be  equally  sharp  when,  as  the  result  of  the 
action  of  the  chemical  rays,  it  is  developed  upon  the 
photographic  plate.  Such  coincidence  of  the  visible 
and  chemical  focus  does  not  exist  either  in  the  telescope 
or  microscope,  but  only  in  the  photographic  lens. 


CONCERNING  LIGHT.  3 

The  optics  of  photography,  therefore,  takes  cog- 
nisance of  rays  transmitted  obliquely  as  well  as  axially, 
and  of  bringing  both  the  chemical  and  visual  rays  to  a 
focus  on  the  same  plane. 

This  paves  the  way  for  a  consideration  of  the 
principles  upon  which  the  various  classes  of  lenses  are 
constructed. 

Concerning  Light. — As  a  fitting  introduction  to  the 
subject  of  lenses,  it  is  necessary  that  an  explanatory 
remark  be  made  on  light.  Without  entering  upon  this 
abstruse  topic,  it  is  enough  for  our  present  purpose  to 
observe  that  the  undulatory  theory  of  light  is  now 
generally  accepted.  This  assumes  light  to  be  a  certain 
result  of  setting  in  motion  the  ether  which  pervades  all 
space,  and  owing  to  that  motion  we  see  objects  upon 
which  such  ether  waves  fall. 

But  the  functions  of  light  are  not  confined  to 
rendering  objects  visible  ;  they  also  include  heating 
and  chemical  action,  or  actinism.  These  three  properties 
of  lighting,  heating,  and  actinism  may  be  very  easily 
demonstrated  by  the  following  simple  experiment  : 
Cover  up  a  south  window  by  an  opaque  screen,  allowing 
the  sun's  rays  to  be  admitted  only  through  a  small 
aperture.  Now  intercept  the  rays  thus  admitted  by  a 
prism,  so  as  to  have  them  spread  out  upon  a  sheet  of 
white  paper,  and  observe  the  gorgeous  spectacle  these 
rays  then  present.  The  beam  of  white  sunlight  is 
decomposed  into  its  primary  constituents,  as  shown 
in  the  diagram  (Fig.  i),  in  which  B  represents  the 
aperture  through  which  the  beam  of  light  is  admitted ; 


PROPERTIES  OF  COLOURED  LIGHT. 


and  which  beam,  but  for  the  interposition  of  the  prism  I' 
would,  without  deviating  from  its  straight  path,  fall  at 
W.  But  the  prism  bends  the  ray,  and  decomposes  il 


W 


FIG.  I. 


into  the  primary  and  secondary  colours  indicated  by  the 
initial  letter  of  the  spectrum,  the  violet  ray  v  having 
been  bent  or  refracted  in  a  greater  degree  than  the  red 
ray  R,  from  which  circumstance  the  violet  and  blue  rays 
of  the  spectrum  are  popularly  spoken  of  as  the  visible 
rays  of  greatest  refrangibility. 

Temonstration  of  Properties  of  Coloured  Light. — 
If  a  strip  of  sensitised  paper  be  pinned  up  so  as  to 
receive  the  spectrum  it  will  soon  be  found  that  it 
becomes  dark  ;  but  the  darkening  power  of  the  light  is 
confined  to  the  rays  at  and  beyond  the  violet  end.  If, 
however,  a  thermometer  be  placed  at  the  various  colours 
of  the  spectrum,  the  mercury  will  rise  in  the  most  pro- 
nounced manner  at,  and  even  beyond,  the  red  end  ; 
hence  the  application  of  the  term  '  heat  rays '  to  these. 


REFRACTION  INFLUENCED  BY  DENSITY.         $ 

That  the  yellow  is  the  luminous  or  light-giving  ray  is 
sufficiently  demonstrated  by  the  sense  of  sight.  Now, 
while  the  foregoing  is  correct  in  the  popular  significa- 
tion, it  is  also  the  case  that  all  the  rays  induce  chemical 
change,  and  it  is  possible  to  prepare  a  sensitive  surface 
upon  which  the  red  rays  will  exercise  more  prompt 
action  than  the  so-called  actinic  or  violet  light.  But 
this  need  not  here  be  considered. 

We  shall  here  sum  up  the  truth  or  law  to  be  deduced 
from  what  has  been  said.  Light  always  travels  in  a 
straight  line  as  long  as  the  density  of  the.  transparent 
medium  through  which  it  is  passing  remains  unchanged. 
Upon  entering  a  denser  medium  obliquely  it  suffers 
refraction  or  bending,  the  amount  of  the  refraction 
depending  altogether  upon  the  density  of  the  medium. 
Pure  water  refracts  more  powerfully  than  air;  water 
containing  a  salt — such  as  nitrate  of  silver — in  solution 
exceeds  pure  water  in  its  refractive  power ;  crown  glass 
exceeds  salted  water,  and  is  in  turn  exceeded  by  flint 
glass,  which  last  must  yield  the  palm  to  the  diamond 
and  other  gems.  Suppose,  then,  we  had  four  simple 
lenses,  all  precisely  alike,  so  far  as  curvature  and  out- 
ward form  were  concerned,  but  one  of  them  was  formed 
by  water  encased  by  glass  shells,  the  others  being  made 
of  crown  glass,  flint  glass,  and  diamond,  respectively ; 
each  would  have  a  different  focus  from  the  other,  the 
water  having  the  longest  and  the  diamond  the  shortest. 

Optical  glass  of  greater  density  is  being  utilised  at 
the  present  time  much  more  extensively  for  photo- 
graphic lenses  than  it  was  several  years  ago.  One 


6  TOPAZ  LENSES. 

practical  advantage  arising  from  this  may  be  perceived 
from  the  principles  just  enunciated.  It  is  this  :  that  with 
a  given  diameter  and  form  of  lens  it  is  possible  to  obtain 
a  shorter  focus,  and,  consequently,  greater  intensity  of 
illumination  than  when  the  objective  is  formed  of  lighter 
materia]. 

Pebble  Lenses. — Transparent  pebbles,  such  as  the 
Brazilian  topaz  and  other  similar  crystalline  bodies  of 
which  spectacle  glasses  are  sometimes  formed,  have  in 
former  times  been  strongly  recommended  as  media  for 
the  construction  of  portrait  lenses.  Sir  David  Brewster 
advocated  this  on  account  of  the  greater  softness  obtained 
by  a  single  lens  of  this  nature  than  by  an  achromatic 
lens.  But  we  now  know  that  the  softness  desiderated 
arose  from  un corrected  aberration,  and  not  from  the 
material  of  which  the  lens  was  formed. 

Aberration— What  is  it? — Seeing  that  throughout  this 
work  there  will  necessarily  be  much  said  concerning  the 
aberration  of  lenses,  it  is  well  here  to  give  such  a  general 
definition  of  the  term  as  will  embrace  the  ramifications 
afterwards  to  be  specially  treated  under  their  proper 
headings. 

Aberration  merely  denotes  that  deviation  of  the  rays 
of  light,  when  inflected  by  a  lens,  whereby  they  are  pre- 
vented from  meeting  in  the  same  point  or  geometrical 
focus.  It  is  of  a  two-fold  nature  :  (a)  that  arising  from 
the  figure  of  the  glass,  and  (b*)  that  caused  by  the  un- 
equal refrangibility  of  the  rays  of  light.  The  former  is 
'  spherical,'  and  the  latter  '  chromatic '  aberration. 


CHAPTER  IL 

PHOTOGRAPHIC  DEFINITION,  REAL  AND  IDEAL — FORMS 
OF  SINGLE  AND  ACHROMATIC  LENSES. 

SHARP  definition  being  an  essential  requisite  in  a 
photographic  lens,  we  shall  here  make  some  observations 
on  this  quality,  and  try  and  assign  a  place  to  the  well- 
defined  photographic  image.  For  reasons  which  will  be 
adduced  we  are  unable  to  give  it  a  higher  than  a  third 
place. 

Ideal  Definition.  —  Definition  of  the  first  order  is 
ideal,  existing  only  in  imagination.  It  is  that  kind 
of  definition  which  presupposes  perfection  in  mathe- 
matical principles,  in  mechanics,  and  in  atmospheric 
conditions.  It  is  tolerant  of  things  as  they  exist, 
merely  because  they  cannot  be  helped.  Optical  tran- 
scendentalism, when  indulged  in  by  the  photographer 
demands  a  lens  which  shall  define  so  perfectly  that  the 
application  of  unlimited  magnifying  power  will  only 
serve  as  a  means  of  unlimited  penetration  into  Nature's 
arcana  ;  a  lens  having  an  aperture  abnormally  great  in 
proportion  to  its  focus,  with  a  range  of  lateral  definition 
so  extensive  as  to  include  a  panorama  ;  and  a  penetrative 
depth  sufficient  to  embrace  everything  from  within  a 
few  feet  to  infinity.  This  is  the  ideal  or  hypothetic 


3  DEFINITION,  REAL  AND  IDEAL 

lens.  Optical  conservatives  say  that  such  a  lens  cannot 
possibly  exist  save  in  the  brain  of  some  enthusiast ; 
but  recent  progress  made  in  Jena,  in  the  production 
of  glass  having  wonderful  and  valuable  optical  pro- 
perties, warrant  us  in  being  very  cautious  in  assigning 
a  limit  to  the  capabilities  of  any  lenses  yet  in  futuro. 
For  its  productions,  however,  when  they  come,  we 
reserve  the  first  place  in  our  classification. 

Telescopic  Definition. — The  second  order  of  definition 
is  that  which  we  find  existing  in  a  well-constructed 
telescope  or  microscope.  The  image  formed  by  their 
object-glasses  is  never  examined  by  the  unaided  eye, 
but  invariably  through  powerful  magnifying  glasses, 
technically  known  as  '  eye-pieces,'  or  l  oculars.'  This 
demands  a  perfection  of  definition  altogether  unknown 
and  unrequired  in  artistic  photography. 

Photographic  Definition. — Definition  of  the  third  order 
is  of  a  lower  grade  than  that  just  described.  Photo- 
graphic definition  may  be  considered  as  fulfilling  every 
requirement  of  our  art-science,  when  not  only  is  there  no 
portion  of  the  picture  noticeably  deficient  in  sharpness, 
even  at  its  margin,  but  also  when  it  bears  the  test  of 
examination  by  a  glass  magnifying  three  or  four  times. 
There  are  many  otherwise  excellent  lenses  which  will 
not  permit  of  this  last  test  being  applied  to  their  pro- 
ductions unless  when  used  with  a  very  small  diaphragm, 
and  it  is  sometimes  desirable  that  one  should  have  the 
power,  both  with  single  and  combination  objectives,  of 
reproducing  a  scene  or  subject  with  less  sharpness  than 
that  which  it  appears  to  possess  to  the  eye  of  the 


CHROMA  TIC  ABERRA  TION.  9 

observer.     The  appliances  for  obtaining  such  effects  will 
be  considered  in  a  subsequent  chapter. 

Kefraction  by  Lenses. — We  have  seen  in  Fig.  I,  Chapter 
I.,  in  what  manner  a  ray  of  light  becomes  decomposed 
when  it  is  transmitted  through  a  prism.  Now,  a  lens 
may  be  considered  a  series  of  prisms  formed  by  a 
single  piece  of  glass,  its  faces  being  spherical  instead  of 
an  unlimited  number  of  flat  surfaces.  The  property 
possessed  by  a  wedge-shaped  piece  of  glass  of  bending 
and  decomposing  a  ray  of  light  applies  equally  to  the 
glass,  whether  it  be  purely  prismatic  or  lenticular  in 
form,  and  no  single  l?ns  formed  of  one  piece  of  glass 
can  possibly  bring  the  rays  transmitted  through  it  to 
one  focus  ;  for,  as  we  have  shown,  the  violet  rays,  being 
bent  so  much  more  strongly  than  the  red  and  all  the 
others,  are  brought  to  a  focus  nearer  to  the  lens  than 
these.  This  defect  is  entitled  'chromatic  aberration,' 
from  chroma  (colour)  and  aberro  (I  wander  from).  Its 
nature  is  shown  in  the  diagram,  Fig.  2,  which  represents 
rays  a  a,  incident  upon  a  double-convex  lens  L.  These 


FIG.  2, 


rays   are  not   only  bent  or  refracted  but  are  also  de- 
composed,  which    is    what    we    have    to    do   with    at 


to  SINGLE  AND  ACHROMATIC  LENSES. 

present.  The  violet  rays,  in  consequence  of  their 
greater  refrangibility,  are  brought  to  a  focus  at  V,  the 
red  rays  finding  a  focus  at  R.  By  the  term  'focus' 
is  here  meant  that  place  where  rays  cross  the  axis  c 
of  the  lens.  This  definition  is  only  strictly  accurate 
when  applied  to  direct  rays ;  a  more  comprehensive  one 
will  be  given  when  we  come  to  treat  of  oblique  pencils. 

Chromatic  aberration  is  avoided  by  the  employment 
of  an  achromatic  (without  colour)  lens.  The  construction 
of  an  achromatic  lens  is  based  upon  the  fact  that  flint 
glass  effects  a  much  greater  separation  of  the  elementary 
colours  of  a  ray  of  light  than  crown  glass.  A  convex 
lens  of  the  latter  material  would,  undoubtedly,  cause  the 
rays  to  be  decomposed,  as  shown  in  Fig.  2,  but  by  being 
placed  in  juxtaposition  with  a  concave  lens  formed  of 
flint  glass,  the  refracting  power  of  which  is  exerted  in 
a  contrary  direction  while  its  power  for  dispersion  is 
greater,  the  inward  dispersive  tendency  of  the  crown  is 
opposed  by  the  outward  dispersive  proclivity  of  the  flint, 
the  result  being  that  the  ray  is  transmitted  intact,  or 
without  colour,  to  its  focus. 

Forms  of  Single  Lenses.  —  In  Fig.  3  are  shown,  in 
outline,  various  forms  of  simple  lenses,  the  names  given 
having  reference  to  the  external  configuration  of  the 
lens,  no  matter  of  how  many  elementary  parts  of  other 
forms  it  may  be  composed. 

In  this  diagram,  I  and  2  are  respectively  plano- 
convex and  plano-concave  lenses  ;  3  and  4  are  double 
convex  and  double  concave ;  5  is  a  concavo-convex  ; 
and  6  a  periscopic  or  meniscus  lens.  If  3  had  one  of 


SI. VOLE  AND  ACHROMATIC  LENSES. 


ii 


its  surfaces  of  greater  curvature  than  the  other,  it  would 
be  designated  a  '  crossed  '  lens. 


FIG.  3. 


When  lenses  are  achromatised  by  uniting  a  convex 
crown  glass  with  a  concave  formed  of  flint  glass,  Fig.  4 


FIG.  4. 

indicates   some   of  the   ways  by  which    such  union   is 
effected. 

Besides  these,  in  which  the  achromatism  is  obtained 
by  the  union  of  one  crown  glass  with  one  flint  glass  lens, 
the  method  (first  applied  to  the  telescope  by  John 
Dollond)  of  uniting  two  crowns  with  one  flint  has  been 
advantageously  applied  to  photographic  lenses,  details 
of  which  will  be  subsequently  given. 


CHAPTER  III. 

THE  CAUSE  OF  AN  INVERTED  IMAGE. 

HAVING  spoken  of  the  nature  of  lenses  we  next  advert 
to  their  properties,  particularly  to  that  special  character- 
istic upon  which  depends  the  formation  of  an  image. 

If  a  double  convex  lens  formed  of  one  piece  of  glass, 
such  as  a  hand  magnifier  of  the  simplest  kind,  be  held 
up  so  as  to  allow  the  sun's  rays  to  be  transmitted  on  to 
a  sheet  of  paper  held  at  a  certain  distance  behind  where 
the  rays  come  to  a  point,  the  brightness  at  the  apex  of 
the  cone  is  owing  to  the  formation  of  a  minute  image  of 
the  sun  there,  its  intensity  either  for  luminousness  or 
burning  being  dependent  upon  the  dimensions  of  the 
lens.  This  applies  also  to  the  formation  of  an  image 
of  any  terrestrial  object  to  which  the  lens  may  in  like 
manner  be  directed.  In  every  case  in  which  an  image 
is  produced  in  this  way  it  will  be  seen  to  be  inverted,  or 
upside  down.  Why  this  is  so  we  shall  explain  by  the 
aid  of  the  following  diagram  (Fig.  5),  in  which  the 
dart  A  may  be  considered  as  representing  anything  in 
external  nature,  such  as  a  church,  a  house,  a  landscape, 
or  a  figure.  The  rays  of  light  from  every  point  of  this 
pass  in  straight  lines  everywhere,  and  hence  through 
the  small  hole  B  in  the  opaque  sheet,  which  may  be 


PINHOLE  APERTURES. 


assumed  to  be  the  front  of  a  box  ;  some  of  these  rays 
from  every  point  pass  straight  on  until  interrupted  by 


FIG.  5. 

the  screen  c,  on  which  they  fall,  forming  an  inverted 
image  of  the  object  in  front. 

Pinhole  Apertures. — The  smaller  the  aperture  B  is 
the  sharper  will  be  the  image.  It  is,  therefore,  quite 
possible  to  take  a  photograph  without  any  lens  what- 
ever ;  but,  owing  to  the  attenuation  of  the  light  by 
transmission  through  a  pinhole  aperture,  a  protracted 
exposure  is  required  in  order  to  obtain  a  picture.  By 
greatly  enlarging  the  aperture  and  inserting  a  lens, 
however,  it  will  be  found  that,  while  the  dimensions  of 
the  image  formed  by  the  pinhole  aperture  are  not 
sensibly  altered,  there  is  at  once  a  great  increase  in 
both  the  brightness  and  sharpness  of  such  image.  It 
may  here  be  remarked  that  the  size  of  the  image  is 
determined  by  the  distance  at  which  the  receiving  screen 
upon  which  the  image  is  depicted  is  situated  from  the 
aperture — a  fact  that  will  be  self-evident  on  inspecting 
the  foregoing  diagram,  and  imagining  the  situation  of 
the  screen  C  to  be  only  half  the  distance  from  the  pin- 
hole  at  which  it  is  now  represented. 

Size  of  Image  determined  by  Focus  of  Lens.  —  From 
what  has  been  said  it  will  be  seen  that  the  longer  the 


14       ANGLE  OF  VIEW  DETERMINED  BY  FOCUS. 

focus  of  a  lens  by  which  an  image  is  to  be  formed  the 
larger  will  be  that  image.  If  a  lens  of  ten  inches  focus 
be  employed  in  the  production  of  a  picture  of  a  scene, 
such  as  a  house  and  its  surroundings,  and  another  picture 
of  the  same  scene  be  taken  by  a  lens  of  five  inches  focus, 
when  both  are  examined  side  by  side  it  will  be  observed 
that  the  house  produced  by  the  lens  of  the  shorter  focus 
will  only  be  one-half  the  dimensions  of  that  obtained 
by  the  lens  of  longer  focus  ;  but,  as  a  set-off  against  this, 
there  will  be  twice  as  much  of  the  subject  depicted  on 
a  plate  the  same  number  of  inches  in  dimension.  From 
this  it  will  be  correctly  inferred  that  a  wide-angle  lens— 
that  is,  a  lens  intended  to  include  a  wide  angle  or  large 
amount  of  the  subject  to  be  photographed  —  must  be  of 
short  focus  relatively  to  other  lenses.  Another  deduction 
from  this  is  that  dimension  or  size  of  image  depends 
exclusively  upon  the  focus  of  the  lens,  and  is  entirely 
unconnected  with  its  diameter.  If  we  have  a  lens  of 
ten  inches  focus  and  only  one  inch  diameter,  and  another 
lens  the  same  focus  and  four  inches  in  diameter,  the 
images  formed  by  them  will  be  precisely  alike  in  di- 
mensions. The  influence  of  the  diameter  of  the  lens 
is  confined  to  giving  greater  or  less  brightness  to  the 
image,  and  we  shall  consider  this  more  fully  when 
treating  of  the  requirements  of  quick-acting  lenses. 


CHAPTER  IV. 

SPHERICAL  ABERRATION. 

A  SINGLE  lens  of  the  class  of  which  we  have  been 
hitherto  treating  does  not  give  an  image  possessing 
more  than  a  very  low  degree  of  sharpness,  even  to  the 
unaided  eye.  This  arises  from  spherical  aberration, 
which  we  may  define  as  an  inability  in  a  lens  having 
a  spherical  surface  to  bring  to  one  focus  all  the  rays 
which  are  transmitted  through  it.  A  ray  transmitted 
by  the  margin  of  a  lens  (Fig.  6)  is  more  deflected 
0 


FIG.  6. 

or  refracted  than  one  which  is  transmitted  nearer  the 
centre.  Observe  in  what  manner  the  representative 
rays  O  and  S  are  refracted  by  the  lens.  The  former, 
being  bent  in  a  greater  degree  than  the  latter,  comes 
to  a  focus  at  o',  the  focus  of  S  being  carried  farther  to 


i6 


SPHERICAL  ABERRATION. 


S' ;  and  the  absolute  focus  of  such  a  lens  will  be 
nowhere  in  particular,  but  anywhere  between  o'  and 
where  the  rays  which  are  more  nearly  central  cross 
the  axial  line.  Now,  this  has  no  connexion  whatever 
with  the  aberration  of  colour,  but  is  true  of  a  lens 
even  if  achromatised.  It  is  possible  to  correct  a  single 
achromatic  lens  so  that  it  shall  with  its  full  aperture 
bring  direct  rays  to  a  focus,  which  is  the  case  with 
telescope  lenses ;  but  for  oblique  rays  it  would  be  quite 
worthless.  Photographic  correction  of  lenses,  therefore, 
partakes  of  the  nature  of  a  compromise  ;  it  is  content 
with  an  inferior  order  of  axial  definition  in  order  to 
secure  an  equal  degree  of  oblique  sharpness. 

A  plano-convex  lens,  or  one  of  a  slightly  meniscus 
form,  if  directed,  convex  side  out,  to  an  object  will  give 
a  fairly  well-defined  image  of  what  is  directly  in  front  : 


FIG,  7. 


but    those   objects    not    axially    situated    will    be    very 
imperfectly    rendered    indeed.      Now,  by  reversing  the 


POSITIVE  SPHERICAL  ABERRATION. 


position  of  the  lens — that  is,  placing  its  flat  side  out- 
wards—  quite  a  different  aspect  is  presented  ;  for  the 
central  sharpness  now  gives  place  to  a  certain  kind  of 
indistinctness  of  image  inferior  in  this  respect  to  the 
former  crispness  of  delineation  ;  but  this  inferior  dis- 
tinctness is  distributed  over  a  larger  area  of  the  plate. 
The  reason  for  this  will  be  seen  from  an  inspection  of 
the  diagram  (Fig.  7),  in  which  a  few  oblique  rays  are 
represented  before  and  after  transmission.  It  will  be 
perceived  that  V  and  w  suffer  less  refraction  than  Y 
and  z,  and  this  being  the  case  there  is  a  great  degree 
of  confusion  at  the  focus,  which,  as  in  the  former 
instance  adduced  with  the  axial  rays  (Fig.  6),  is  really 
*  nowhere.' 

Positive  and  Negative  Spherical  Aberration. — In  the  fore- 
going instances  and  illustrations,  in  which  the  margin 
of  the  lens  refracts  the  light  to  a  much  greater  extent 
than  does  its  centre,  the  aberration  is  positive.  But  it  is 


FIG.  8.  FIG.  9. 

quite  easy  to  combine  two  glasses,  one  a  convex  and 
the  other  a  concave,  with   an   air-space  between  them 

C 


iS  NEGATIVE  SPHERICAL  ABERRATION. 

in  which  this  condition  will  be  reversed,  or,  to  put  it 
popularly,  in  which  the  centre  of  the  compound  will  be 
possessed  of  a  great  magnifying  power  and  the  margin 
not  necessarily  any  at  all.  The  two  illustrations  here 
given  (Fig.  8  and  Fig.  9),  in  which  the  inner  surfaces 
are  of  dissimilar  radii  of  curvature,  afford  a  fair  idea 
of  the  conditions  requisite  to  attain  this  end.  This 
property  is  known  as  negative  spherical  aberration,  and 
its  use  in  flattening  the  field  of  certain  combinations  will 
hereafter  be  pointed  out. 

No  single  lens  can  be  made  that  shall  be  entirely 
free  from  spherical  aberration,  but  by  giving  a  lens  a 
certain  form  it  may  be  very  greatly  reduced.  If  a  lens 
be  a  plano-convex,  and  its  flat  side  be  directed  towards 
the  object,  the  aberration  is  4*5  ;  but  if  the  position  is 
reversed,  and  the  convex  side  held  toward  the  object, 
the  aberration  is  reduced  to  ri/. 

The  longitudinal  aberration  is  ascertained  by  noting 
the  difference  between  the  focus  given  by  the  margin  of 
a  lens  and  that  of  its  middle.  While  making  this  trial, 
opaque  masks  must  be  employed  to  prevent  the  trans- 
mission of  light  through  any  but  the  part  being  tested. 

In  a  lecture  on  lenses  at  the  Society  of  Arts,  Mr. 
Conrad  Beck  gave  '  in  a  nutshell '  a  synopsis  of  the 
aberrations  of  the  various  forms  of  lenses,  both  convex 
and  concave.  Premising  that,  as  a  general  rule,  when 
parallel  rays  enter  from  a  less  refracting  medium  (air) 
into  a  denser  medium  (glass),  the  more  curved  the 
surface  that  is  turned  towards  the  parallel  rays  the 
less  is  the  aberration,  while  the  flatter  the  curve  or 


ABERRA  TION  OF  SINGLE  LENSES. 


the  more  nearly  it  approaches  a  flat  surface  the  greater 
the  aberration,  the  amount  of  such  aberration  is  shown 
in  the  following  figures,  the  parallel  rays  being  assumed 
to  enter  each  individual  lens  from  the  left-hand  side. 


S.Aberraticn 

4-2-07  +1-071 


.1-17 


S.  Aberration 
-1071 

FIG.  10. 


-2-07 


-4'5 


In  the  above,  the  upper  or  convex  series  are  of 
the  same  focus  as  the  lower  or  concave  series,  so  that 
any  one  of  the  former  will  just  balance  that  of  the 
negative  focus  below.  The  amount  of  aberration,  plus 
or  minus,  is  placed  underneath  each.  By  comparing  the 
figures  attached  to  any  of  these  lenses,  even  those  of  the 
same  form,  such  as  the  first  and  last  in  upper  series,  it 
will  be  perceived  to  what  extent  aberration  is  affected 
according  to  the  side  which  is  turned  towards  the  light. 


CHAPTER  V. 

THE  NA'iURE  AND  FUNCTION  OF  THE  DIAPHRAGM 
OR  STOP. 

How,  by  whom,  or  at  what  time  a  diaphragm  came 
to  be  designated  a  '  stop '  we  need  not  here  wait  to 
inquire.  Photography  has  given  rise  to  so  many  new 
terms  and  new  applications  of  pre-existing  terms  that  its 
literature,  and  especially  its  vernacular  dicta,  must  not 
be  considered  as  amenable  to  strict  etymological  rules. 
A  diaphragm,  in  all  other  branches  of  optical  science 
than  that  of  photography,  differs  from  a  stop,  but  in 
our  young  art-science  they  are  held  by  the  vox  populi 
to  be  synonymous ;  hence  the  indiscriminate  employ- 
ment of  the  two  terms  in  what  we  have  further  to  say  in 
these  chapters. 

Use  of  a  Diaphragm. — A  diaphragm  fulfils  two  alto- 
gether dissimilar  functions  in  photography,  according  to 
whether  the  lens  to  which  it  is  attached  be  a  single  or  a 
compound  instrument.  In  the  former  it  is  usually  a  neces- 
sity ;  in  the  latter  only  an  expedient.  It  has  been  shown 
in  what  manner  rays  are  transmitted  by  a  single  lens,  and 
that  those  impinging  upon  one  part  of  the  surface  are 
not  brought  to  a  focus  with  such  rays  as  are  permitted 
to  fall  upon  another  portion.  Now,  by  placing  a 
diaphragm  at  a  little  distance  in  front  of  the  lens,  it 


DIAPHRAGMS  OR  STOPS. 


21 


cures  all  the  evils  arising  from  spherical  aberration  by 
debarring  access  to  those  rays  which,  if  transmitted, 
would  interfere  with  ultimate  sharpness. 

In  Fig.  1 1  we  show  in  what  manner  the  '  curative ' 


FIG.   II. 


powers  of  the  diaphragm  are  exercised  when  employed 
as  a  stop  to  obstructant  rays,  both  central  and  oblique. 
Observe  what  havoc  would  be  played  as  regards  focal 
sharpness  if  the  mass  of  the  rays  were  permitted  indis- 
criminate access  to  the  lens.  The  dotted  lines  represent 
those  by  which  definition  would  be  entirely  marred 
•vere  they  not  stopped  by  the  diaphragm,  which,  sentry- 
like,  guards  the  access  to  the  lens.  What  has,  therefore, 
to  be  effected  in  this  case  by  the  diaphragm  is  this — 
no  rays  are  allowed  to  take  part  in  the  formation  of  the 
central  portion  of  the  picture  but  those  transmitted 
through  the  centre  of  the  lens  ;  and,  in  like  manner, 


22     MISCONCEPTIONS  REGARDING  DIAPHRAGMS. 

none  but  rays  transmitted  through  the  margin  of  the 
lens  are  allowed  to  form  any  but  the  margin  of  the 
picture.  This  is  the  law  regulating  the  margin  of  a 
diaphragm  to  a  single  achromatic  lens,  and  from  what 
has  been  said  it  will  be  seen  that  to  a  lens  of  this  class 
the  stop  is  a  necessity. 

Misconceptions  Regarding  Diaphragms. — Before  pro- 
ceeding further  we  may  allude  to  a  very  prevalent  and 
popular  misconception,  which  finds  expression  in  the 
suggestion  that  by  making  the  lens  of  only  the  diameter 
of  the  largest  diaphragm  an  equal  degree  of  sharpness 
would  be  secured.  While  this  is  quite  true  as  regards 
the  formation  of  the  centre  of  the  picture — which  would 
be  equally  well  defined  if  an  opaque  disc  of  paper  having 
a  round  hole  in  its  centre  were  pasted  upon  the  surface 
of  the  lens,  and  by  which  it  would  be  practically  reduced 
to  the  dimensions  of  the  aperture  in  the  paper — it  is  not 
so  with  the  sides  of  the  picture,  which,  although  equally 
well  lighted  as  before,  are  now  badly  defined.  The 
following  experiment  is  both  suggestive  and  instructive  : 
—Let  a  plano-convex  or  meniscus  lens  (the  front  lens  of 
a  portrait  combination  answers  the  purpose  well)  be 
mounted,  flat  side  out,  and  without  any  diaphragm. 
Now  try  to  focus  the  image,  and  observe  that  while  no 
part  of  it  is  sharp,  it  is  rather  more  so  in  the  centre  than 
towards  the  sides.  Next  make  a  cardboard  diaphragm, 
with  an  aperture  about  one-fourth  the  diameter  of  the 
lens,  push  it  close  up  against  the  flat  surface,  and  then 
focus  the  centre  as  sharply  as  possible.  This  will  now 
be  well  defined,  but  only  over  a  very  limited  area. 


DIAPHRAGMS  CONFER  SHARPNESS.  23 

Without  altering  the  camera  or  lens  pull  the  diaphragm 
slowly  away  from  the  lens,  and  it  will  be  found  that,  by 
the  simple  act  of  increasing  the  space  between  the 
diaphragm  and  the  lens,  the  area  of  sharpness  extends 
outwards,  till  a  point  is  reached  at  which  further  with- 
drawal of  the  diaphragm  cuts  off  the  light  from  the 
corners  of  the  plate  without  further  increasing  the 
marginal  definition.  At  this  stage  the  requirement  has 
been  fulfilled  that  the  centre  of  the  picture  be  formed 
by  the  centre  of  the  lens,  and,  in  like  manner,  that 
no  rays  have  taken  part  in  the  formation  of  the 
margins  of  the  picture  but  those  transmitted  by  the 
margin  of  the  lens. 

When  applied  to  a  combination  of  lenses — such  as 
that  employed  in  portraiture — the  function  of  the  dia- 
phragm is  different  from  that  just  described  ;  for  such 


FIG.  12. 


combination,    being    corrected    in    itself    for    spherical 
aberration,    gives  a  sharp  image  with  its  full  aperture. 


24  DIAPHRAGMS  GIVE  PENETRATION. 

But  it  is  characteristic  of  all  portrait  lenses  and  others 
having  a  large  working  aperture  that  they  lack  the 
power  of  bringing  objects  situated  at  different  distances 
to  a  focus  on  one  plane  ;  or,  as  it  is  commonly  said,  they 
have  no  '  depth  of  focus.'  By  reducing  the  aperture  a 
portrait  lens  can  be  made  to  possess  as  much  of  this 
depth  of  defining  power  as  may  be  required.  The  way 
by  which  this  is  secured  is  shown  in  the  diagram,  Fig.  12, 
in  which  the  dotted  lines  A  A  represent  the  rays  trans- 
mitted through  a  lens  worked  with  its  full  aperture. 
Observe  that  this  focus  partakes  of  the  nature  of  a  definite 
point,  at  which  it  is  imperative  that  the  ground  glass  of 
the  camera  be  situated  in  order  to  obtain  sharpness. 
Now  this  is  all  very  well,  and  it  is  the  easiest  thing  in 
the  world  to  place  the  focussing-screen  in  that  precise 
position.  But  here  lies  the  difficulty  :  this  spot  of  precise 
focus  is  that  for  rays  only  which  come  from  a  definite 
distance  in  front  of  the  lens  (say  12  feet),  while 
those  rays  emitted  by  an  object  either  ten  or  fourteen 
feet  away  do  not  focalise  at  the  same  point  or 
distance  behind  the  lens — one  set  of  rays  coming  to 
a  focus  nearer  and  the  other  further  than  the  twelve- 
feet  set. 

To  meet  the  difficulty  just  stated  we  must  have 
recourse  to  a  diaphragm  by  which  all  rays  outside  of 
SS  are  excluded,  with  this  result  —  that  the  point  at 
which  the  rays  crossed  the  axis  of  the  lens  has  now  in 
effect  become  elongated,  and  a  fairly  good  focus  is 
obtained  without  the  necessity  that  formerly  existed  for 
having  the  ground  glass  situated  in  one  definite  position. 


DIAPHRAGMS  INCREASE  LA  TERAL  DEFINITION.   25 

The  reduction  of  the  aperture  has  given  such  a  range  to 
the  focus  that,  while  the  sharpness  of  the  object  originally 
focussed  upon  with  full  aperture  remains  unimpaired, 
this  quality  is  now  imparted  to  objects  situated  both 
nearer  to  and  further  from  the  camera. 

No  *  Depth  of  Focus'  in  Large  Portrait  Lenses. — For 
the  reason  just  given  a  portrait  lens  of  very  large 
dimensions  cannot  be  used  with  its  full  aperture  in 
taking  a  head  unless  the  sitter  be  placed  a  considerable 
distance  from  the  instrument,  because  such  is  the  lack 
of  depth  of  definition  in  a  lens  of  this  character  that  if 
the  nose  were  sharply  focussed,  the  eye,  ear,  and  other 
portions  not  situated  upon  the  plane  'of  the  nose  would 
be  so  much  out  of  focus  as  to  destroy  the  pictorial  value 
of  the  head;  while  by  focussing  merely  the  eye,  the 
nose  and  chin  would  be  equally  out.  By  employing  a 
diaphragm,  however,  all  the  features  may  be  brought 
into  pictorial  sharpness. 

We  may  here  foreshadow  what  we  shall  have  to  say 
afterwards  in  its  proper  place  relative  to  the  use  of  stops, 
by  observing  that  a  portrait  or  aplanatic  lens  (an 
aplanatic  lens  being  one  which  is  capable  of  working 
with  full  aperture)  not  only  has  its  focal  range,  as 
regards  depth,  increased  ad  libitum  by  the  employ- 
ment of  a  diaphragm,  but  it  has  its  lateral  definition 
improved  in  similar  ratio.  A  lens  when  worked  with 
full  aperture  is  unsuited  for  photographing  anything 
requiring  great  marginal  sharpness,  such  as  copying  a 
large  sheet  of  printed  matter  or  photographing  a  house 
pn  a  plate  otherwise  within  its  capacity.  By  inserting  a 


26    FOCUSSING  WITH  THE  WORKING  DIAPHRAGMS. 

diaphragm  the  range  of  sharpness  will  be  so  far  extended 
as  to  enable  the  lens  to  execute  work  for  which,  without 
having  recourse  to  this  expedient,  it  would  have  been 
altogether  unsuited. 

Focussing  with  the  Working  Stop. — Unless  a  lens  be 
quite  free  from  spherical  aberration,  or,  in  other  words, 
be  aplanatic,  it  is  well  to  focus  with  the  same  stop  with 
which  the  picture  is  to  be  taken.  There  is  often  a  great 
temptation  to  focus  with  a  large  diaphragm  on  account 
of  the  superior  illumination  of  the  image  thus  obtained, 
and  then  insert  a  small  one.  But  with  some  lenses  this 
ensures  the  very  evil  it  is  intended  to  avoid,  for  with  a 
small  stop  the  best  point  of  focus  is  farther  from  the  lens 
than  when  employing  a  large  one.  The  reason  for  this 
will  be  apparent  on  studying  the  diagrams,  Figs.  6  and  7. 

For  composing  a  picture,  when  one  cannot  have  too 
much  light  upon  the  focussing  screen,  it  may  be  well  to 
employ  the  largest  aperture  possible ;  but  when  the 
subject  has  been  arranged,  then  should  the  focussing  be 
done  as  above  indicated. 


CHAPTER  VI. 

PROPERTIES  OF  DEEP  MENISCUS  LENSES-- 
COMPENSATING SINGLE  LENSES. 

THE  simpler  the  parts  and  structure  of  a  photo- 
graphic objective  the  less  danger  is  to  be  apprehended 
from  flare  or  false  light  caused  by  internal  reflections. 
This  being  the  case,  why,  it  may  be  asked,  not  employ 
the  simplest  of  all  lenses — a  single  meniscus  ? 

The  Deep  Meniscus. — A  deep  meniscus  lens,  whether 
single  or  achromatic,  possesses  properties  different  from 
all  others.  Those  who  desire  to  see  the  finest  exemplifi- 
cation of  the  so-called  *  depth  of  focus '  possible  to  be 
obtained  have  only  to  procure  a  meniscus  of  very  deep 
shape,  expose  its  concave  side  to  a  bright  object,  and 
observe  the  image.  This  experiment  may  be  performed 
by  directing  it  to  the  flame  of  a  candle  situated  at  a 
distance  of  a  few  yards  and  receiving  the  image  on  a 
sheet  of  paper  held  in  the  hand.  Having  got  the 
sharpest  image  that  can  be  obtained,  observe  to  what  a 
great  extent  the  lens  may  be  moved  backwards  and 
forwards  without  the  identity  of  the  candle  flame  ceasing 
to  be  observed.  It  is  true  that  it  is  surrounded  with  an 
aureola  of  false  light,  but  the  form  itself  is  still  there.  In 
this  respect  it  is  quite  unlike  an  image  obtained  by  any 
other  lens,  such  as  a  plano-convex,  curved  side  out,  in 


28  DEEP  MENISCUS  LENSES. 

which  the  slightest  motion  of  the  lens  from  its  correct 
focal  distance  converts  the  image  of  the  flame  into 
a  circular  disc  of  light. 

The  spherical  aberration  by  which  the  flare  or 
mistiness  of  the  image  in  the  foregoing  experiment  is 
caused  can  be  practically  eliminated  by  the  employment 
of  a  diaphragm  ;  and  here  we  may  observe  that  photo- 
graphs of  great  beauty  and  even  sharpness  may  be,  and 
often  have  been,  taken  by  means  of  a  simple  non-achro- 
matic meniscus  lens.  For  a  reason  which  will  be  apparent 
to  those  who  carefully  study  the  diagram,  Fig.  2  (page  9), 
the  photographic  image  will  not  be  sharp  unless  care  has 
been  taken  that,  after  focussing  upon  the  ground  glass, 
the  lens  is  then  pushed  in  towards  the  camera  to  such 
an  extent  as  to  cause  the  focus  of  the  chemical  or  violet 
rays  to  take  the  place  of  the  visual  ones,  which,  as 
regards  the  ground  glass,  will  now  be  quite  out  of  focus. 
The  difference  between  these  foci  is  approximately  one- 
thirtieth  of  the  focus  of  a  lens  formed  of  crown  glass; 
hence,  if  a  ten-inch  lens  were  employed  it  would,  after 
focussing  sharply,  have  to  be  pushed  in  over  a  quarter  of 
an  inch  in  order  to  secure  a  sharp  image  on  the  sensitive 
plate.  Now,  this  would  be  of  no  consequence  whatever 
if  distant  objects  alone  were  to  be  photographed,  because, 
the  difference  between  the  two  foci  being  a  constant 
one,  the  ground  glass  could  easily  be  let  deeper,  or  set 
farther  forward,  in  its  frame  to  effect  the  requisite  com- 
pensation. But  while  the  difference  is  a  constant  one 
with  respect  to  proportion,  it  is  not  so  as  regards 
quantity;  for  upon  focussing  a.  near  object  the  lens,  as 


COMPENSATING  SINGLE  LEASES.  i$ 

every  one  knows,  must  be  withdrawn  farther  from  the 
focussing-screen  in  order  to  obtain  a  focus,  and  the 
quarter-of-an-inch  alteration  of  the  screen  in  the  frame 
would  prove  totally  inadequate  when,  in  photographing 
an  object  on  the  scale  of  the  original,  the  lens  had  to  be 
twenty  inches  from  the  plate. 

This  would  obviously  demand  an  adjustment  be- 
tween the  visual  and  the  working  focus  of  a  measure- 
ment greatly  exceeding  that  employed  under  the 
circumstances  described.  Among  other  reasons,  the 
trouble  necessitated  in  effecting  this  adjustment  has 
operated  to  prevent  photographers  from  making  use  of 
lenses  other  than  those  in  which  the  actinic  achromatism 
is  effected  in  such  a  manner  as  to  ensure  a  strict  coinci- 
dence of  the  visual  and  chemical  rays.  But  as,  notwith- 
standing the  drawback  mentioned,  there  are  several 
advantages  alleged  to  be  found  in  simple  crown  glass 
meniscus  lenses  —  cheapness  being  one,  and  less  loss 
of  light  another — it  is  fitting  that  we  here  give  the 
means  whereby  an  accurate  adjustment  can  be  made  so 
as  to  ensure  the  requisite  sharpness  with  such  lenses 
when  used  in  either  a  single  or  combined  state. 

Compensating  Methods  for  Simple  Lenses.  —  Propor- 
tional compasses  and  suitable  markings  upon  the  sliding 
mount  will  suggest  themselves  as  one  obvious  method 
by  which  to  effect  the  desired  adjustment ;  but  that 
to  which  we  have  long  confined  ourselves — invariably 
recommended  as  superior  to  all  other  methods,  and 
which  owes  its  inception  to  that  profound  mathematical 
optician,  Mr.  Robert  H.  Bow,  C.E.,  of  Edinburgh — is 


30  COMPENSATING  SIMPLE  LENSES. 

one  more  practically  perfect  (as  we  have  often  proved  it 
to  be  under  many  ramifications)  that  even  its  talented 
progenitor  could  easily  have  imagined  it  to  be.  A  weak 
and  thin  convex  lens — such  as  may  be  obtained  from 
spectacle  lens  opticians — must  be  selected,  its  strength 
being  such  that,  when  added  to  the  focal  length  of  the 
operating  lens,  it  will  have  the  power  of  reducing  the 
focus  two  per  cent,  or  any  other  proportion  found  to  be 
the  proper  amount  of  adjustment  for  a  very  distant 
object.  As  the  focal  length  of  this  supplementary  lens 
will  be  very  great — say  from  forty-five  to  fifty  times  that 
of  the  camera  lens — very  little  error  will  be  caused  by 
inserting  it  at  the  place  of  the  stop  instead  of  in  con- 
tact with  the  working  lens.  It  has,  therefore,  merely  to 
be  dropped  in  a  suitable  slit  in  the  mount,  like  a  Water- 
house  diaphragm,  where  it  remains  till  the  focus  is 
obtained,  after  which  it  is  removed  and  the  photograph 
taken  without  it.  The  simplicity  and  beauty  of  this 
system  must  approve  itself  to  every  one. 

The  rule  for  finding  the  focus  of  the  lens  that  must 
be  inserted  as  a  stop  (when  focussing)  to  effect  the  cor- 
rection of  the  working  lens  is  this — f  being  the  focal 

f  x  f 

length    of   the    required    lens :— -f—~- -^     or     when 

/i  ~/n 

/j  —  50,  /n  =  49,  /-—  2450  inches.  This  rule  will  be 
found  useful  in  another  direction  when  we  come  to 
speak  of  over-corrected  lenses  ;  for  the  means  described 
for  curing  the  annoyances  arising  from  the  use  of  non- 
achromatised  lenses  apply  equally  to  those  in  which  the 
achromatism  for  colour  is  cawed  further  than  is  re- 


DEEP  MENISCUS  LENSES.  3t 

quired  for  photographic  working  as  to  those  in  which  it 
is  not  carried  sufficiently  far. 

Deep  Meniscus  Lenses  require  Small  Diaphragms. — A 
deep  meniscus,  whether  achromatised  or  not,  requires  a 
small  stop  placed  comparatively  close  to  the  lens.  This 
permits  of  the  transmission  of  a  very  oblique  ray,  the 
incidence  of  the  ray  being  more  normal  than  in  the  case 
of  a  flatter  lens.  For  this  reason  all  wide-angle  lenses 
must  partake  of  the  external  form  of  the  deep  meniscus, 
and  the  diaphragms  must  be  placed  near  to  the  lens. 

When  single  meniscus  lenses  are  mounted  in  doublet 
form — that  is,  one  lens  in  front  of  and  the  other  behind 
the  diaphragm  —  there  is  a  help  towards  correction 
accomplished  naturally  in  the  case  of  oblique  rays,  the 
nature  of  which  we  may  explain  as  follows  :  —  Let  a 
symmetrical  or, by  preference, a  non-symmetrical  doublet, 
of  which  the  back  element  is  shortest  in  focus,  be  im- 
agined, its  two  elements  being  deeply  curved  crown- 
glass  menisci.  When  an  oblique  ray  impinges  upon  the 
anterior  lens  in  such  a  manner  as  to  enable  it  to  be 
transmitted  through  a  stop  placed  between  both  lenses, 
it  undergoes  decomposition,  and  its  violet  constituent, 
being  more  strongly  refracted  than  the  yellow,  falls 
upon  the  surface  of  the  posterior  lens  nearer  its  margin 
than  does  the  yellow  ray,  which,  as  we  have  said,  is  less 
refrangible  than  the  other.  But  the  nearer  to  the  centre 
of  a  lens  that  a  ray  falls  for  transmission,  the  less  is 
it  refracted  ;  or,  on  the  contrary,  the  margin  of  a  lens 
possesses  the  refractive  power  in  a  greater  degree.  The 
yellow  and  violet  rays  which,  therefore,  were  separated 


$i  ELASTICITY  OF  FOCtfS. 

by  the  action  of  the  front  lens  are,  to  some  extent,  made 
to  reunite  by  the  back  lens,  seeing  that  the  violet  falls 
under  the  influence  of  a  portion  of  this  back  potent  to 
cause  it  to  reunite  with  the  yellow,  which,  being  less 
refrangible  in  itself,  is  also  transmitted  by  a  portion  of 
the  lens  possessing  less  power  for  refracting. 

Accommodating  Elasticity  of  Focus.  —  The  deep  me- 
niscus lends  itself  wonderfully  to  combinations  intended 
to  have  an  easy,  accommodating  elasticity  of  focus.  A 
single  achromatic,  deep  meniscus,  which  is  properly 
corrected  for  actinic  achromatism,  may  have  wedded  to 
it  as  a  back  combination  a  lens  formed  of  a  single 
crown-glass  meniscus,  which  shall  not  only  correct  the 
distortion  of  figure  necessarily  caused  by  the  former 
when  used  alone,  but  shall  do  so  without  much  inter- 
ference with  its  actinic  correction.  In  other  words,  the 
achromatised  front  when  used  alone  has  its  chemical 
and  visual  foci  coincident ;  yet  when  a  single,  non- 
achromatic,  crown-glass  meniscus  is  added  to  this, 
although  there  is  a  diminishing  of  the  focus  to  about 
one-half,  the  chemical  and  visual  foci  are  still  practically 
coincident  as  before. 

A  practical  outcome  of  this  fact  is  that,  when  a 
photographer  has  a  lens  of  the  achromatised,  wide- 
angle,  non-distorting  class,  which  may  not  be  of  pre- 
cisely the  focus  he  desires,  he  may  temporarily  lay  aside 
its  posterior  element  and  substitute  for  it  a  simple  lens 
of  another  focus,  by  which  he  can  arrive  at  almost  any 
focal  result  required.  Having  determined  upon  the 
focus  desiderated  he  must  start  with  this  fact  as  a  basis 


SEFARA  TION  IN  DETERMINING  FOCUS.          33 

that  no  two  lenses  of  only  half  that  focus  will  enable 
him  to  obtain  what  he  desires.  An  important  factor  in 
the  calculation  is  the  distance  that  must  intervene  be- 
tween the  two  lenses  forming  a  combination.  Knowing 
the  foci  of  the  particular  lenses  about  to  be  employed  in 
the  formation,  temporary  or  otherwise,  of  a  new  objec- 
tive, the  combined  focus  of  the  pair  may  be  ascertained 
by  multiplying  together  the  individual  foci  and  dividing 
by  the  foci  added  together,  subtracting  from  the  divisor 
the  distances  apart  at  which  the  lenses  are  to  be 
mounted. 

It  will  be  obvious  that  when  a  combination  is  very 
near  the  focus  desired,  that  focus  may  be  lengthened  or 
shortened  till  the  required  power  is  obtained  by  slightly 
separating  or  bringing  the  lenses  nearer  together.  The 
nearer  they  are  together,  the  shorter  the  equivalent 
focus. 

This  question  will  be  found  treated  with  greater 
fulness  in  the  chapter  on  '  The  Adjustment  of  Dissimilar 
Lenses/ 


CHAPTER  VII. 

THE  OPTICAL  CENTRE  OF  SINGLE  LENSES. 

PREVIOUS  to  the  consideration  of  either  the  solar, 
the  equivalent,  or  conjugate  foci  of  lenses,  it  is  necessary 
that  we  speak  of  the  'optical  centre,'  this  being  the 
point  from  which  focal  measurements  must  be  made. 
Our  remarks  will,  at  first,  have  reference  only  to  the 
optical  centre  of  a  lens,  by  which  we  mean  just  what  is 
expressed  by  this  name  and  not  of  an  objective  or  com- 
bination of  lenses,  which  is  quite  another  matter ;  for, 
if  one  choose  to  be  too  nice  with  definitions,  it  is  not 
difficult  to  show  that  a  combination  has  not  an  optical 
centre  at  all,  or,  to  put  it  more  intelligibly,  that  any 
given  combination  may  have  its  optical  centre  at  several 
places,  according  to  the  circumstances  under  which  it 
is  being  employed. 

The  situation  of  the  optical  centre  for  focal  centre,  as 
it  has  by  some  been  designated)  of  a  lens  is  determined 
by  its  form.  In  some  forms  it  is  within,  and  in  others 
outside,  of  the  lens.  In  a  double-convex  it  is  in  the 
middle,  or  equi-distant  from  both  surfaces  ;  in  a  crossed 
lens  it  is  situated  at  a  point  between  the  middle  and  the 
more  convex  of  the  two  surfaces  ;  a  plano-convex  has  its 


CENTRE  OF  LENSES.  35 

optical  centre  on  the  curved  surface;  while  in  a  meniscus 
it  is  outside  altogether,  its  distance  from  the  lens  being 
determined  by  the  degrees  of  curvature  of  the  surfaces. 

To  find  the  Centre  of  a  Single  Lens. — The  method  for 
finding  the  optical  centre  of  a  lens  is  this  : — Draw  two 
parallel  radial  lines,  one  from  the  centre  of  each  cur- 
vature, and  both  being  oblique  to  the  axis;  then  connect 
the  points  at  which  they  touch  the  curved  surface  by  a 
line  which,  in  the  case  of  a  meniscus,  must  be  prolonged 
till  it  meets  the  axis.  The  point  at  which  this  junction 
line  touches  the  axis  is  the  optical  centre.  We  shall 
now  illustrate  this  law  by  applying  it  to  the  case  of 
three  of  the  four  lenses  just  named. 

Centre  of  Double  Convex  Lens. — In  Fig.  13  we  have  a 
double  convex  lens,  the  radii  of  curvatures  of  both 
surfaces  being  a  and  a,  the  lines  from  which  to  the 


further  surfaces  in  this  and  the  two  following  figures 
are  parallel  to  each  other.  From  their  points  of  impact 
on  their  respective  surfaces,  as  s  /,  a  connecting  line  is 
drawn,  and  at  the  point  o,  where  this  line  touches  the 
axis,  is  situated  the  optical  centre, 


36  CENTRE  OF  LENSES. 

Centre  of  Crossed  Lens. — By  the  flattening  of  one  of 
the  curves  of  the  lens  it  becomes,  as  in  Fig.  14,  a  crossed 
lens,  having  its  optical  centre  at  0,  which  in  this  case  is 
not  centrally  situated. 


FIG,  14. 

Centre  of  a  Piano-Convex  Lens. —  This  centre  being 
situated  on  the  convex  surface  of  the  lens,  it  is  not 
necessary  here  to  give  an  illustration 

Centre  of  Meniscus. —  It  is  in  the  case  of  the  deep 
meniscus  now  so  much  in  use  for  many  purposes,  both 
singly  and  combined,  where  the  greatest  discrepancy 
exists  between  one's  ordinary  or  crude  conjectures  as  to 
the  situation  of  the  optical  centre  and  its  true  position. 
In  Fig.  15  it  is  demonstrated  not  only  to  be  outside  of 
the  lens,  but  a  long  way  outside.  We  have  heard  the 
question  put  to  one  who  was  reputed  to  be  fairly  con- 
versant with  optical  matters  :  '  Where  must  I  measure 
the  focus  of  my  lens  from  ? ' — the  lens  spoken  of  being 
a  wide-angle,  deep  meniscus,  having  a  stop  in  front. 
The  response  was :  '  You  will  be  sufficiently  ac- 
curate by  measuring  from  the  centre  of  the  curved 
surface  of  the  lens.'  Now,  this  reply  is  not  correct 


PROPERTIES  OF  OPTICAL  CENTRE.  tf 

in  the  case  of  a  lens  of  this  form,  although  it  would 

be   so    if    one   surface   were    plane    instead    of  being 
concave. 


FIG.  15. 

Properties  of  Optical  Centre. — One  of  the  properties 
of  the  optical  centre  of  a  lens  is  this — that  any  ray 
refracted  by  the  lens  which  passes  through  this  centre 
emerges  in  a  direction  parallel  to  that  of  its  incidence. 
In  most  of  the  class-books  on  optics,  the  rule  for  finding 
the  optical  centre  is  expressed  thus :  '  Multiply  the 
thickness  of  the  lens  by  the  radius  of  one  surface  and 
divide  the  product  by  the  sum  of  the  radii,  and  the 
quotient  is  the  distance  of  the  centre  from  the  vertex 
of  that  surface.'  The  position  of  the  centre  is  the  same 
in  every  lens  of  the  same  dimensions,  whatever  may  be 
the  material  of  which  it  consists. 

What  has  hitherto  been  said  applies  to  single  lenses, 
to  which  alone  the  term  'optical  centre'  is  strictly 
applicable ;  and,  although  we  have  confined  the  illus- 
trations to  those  of  the  positive  or  convex  class,  the 
rules  equally  apply  to  concave  lenses. 


CHAPTER  VI1L 

THE  OPTICAL  OR  FOCAL  CENTRE  OF  A  COMBINATION. 

IN  a  combination  of  lenses,  whether  symmetrical  or 
non-symmetrical,  there  is  no  fixed  point  which  can  be 
termed  the  *  optical  centre.'  The  mistake,  however,  is 
frequently  made  of  assigning  it  a  position  where  the 
stop  is  placed.  But  the  best  position  for  the  stop  has 
not  necessarily  any  relation  to  that  of  the  centre,  which 
can  only  have  its  position  determined  upon  knowing 
the  precise  circumstances  under  which  the  combination 
is  to  be  used,  for  it  has  strict  relation  to  the  conjugate 
foci.  If  these  weie  so  definitely  fixed  as  to  be  invariable, 
then  the  position  of  the  centre  could  be  definitely  allo- 
cated, but  not  otherwise  ;  for  every  alteration  of  focus 
would  be  attended  with  a  displacement  of  the  central 
point.  What  is  commonly  termed  the  '  optical  centre ' 
in  a  combination  is  in  reality  the  centre  of  conjugate 
foci,  and  this  is  determined  by  the  conjugates,  which, 
as  already  said,  are  changed  with  nearly  every  change 
of  picture  taken. 

How  to  find  the  Focal  Centre. — The  place  of  the  centre 
in  a  combination  of  any  nature  cr  form  may  be  easily 
found,  and  for  any  special  purpose  it  may  be  marked 
upon  the  brasswork  of  the  me.  anting.  The  method 


TO  FIND  THE  FOCAL  CENTRE.  39 

now  to  be  described  is  one  which  involves  no  special 
apparatus.  Suppose  the  lens  to  be  a  large  portrait 
combination,  and  it  is  desired  to  know  its  centre  when 
employed  in  portraiture.  Let  us  assume  the  anterior 
conjugate  (the  sitter)  to  be  at  an  average  distance  of 
(say)  eighteen  feet  from  the  camera,  then  let  the  lens 
be  brought  into  a  darkened  room  and  placed  upon  a 
board  on  the  table.  On  this  board  must  be  laid  a 
small  square  block  of  wood  about  two  inches  in  height, 
and  the  upper  surface  of  which  is  brought  to  a  wedge 
shape.  Now  rest  the  lens  across  the  face  of  the  wedge, 
and  let  it  be  directed  to  a  lighted  candle  placed  in  front 
at  a  distance  equal  to  that  at  which  the  sitter  is  expected 
to  be  placed,  and  having  erected,  a  few  inches  behind  the 
lens,  a  white  sheet  of  cardboard  on  which  to  receive  the 
image  of  the  candle,  hold  the  lens  (the  weight  of  which 
rests  upon  the  wedge-shaped  block)  level  by  the  fore- 
finger of  the  left  hand,  and  with  the  right  hand  rotate 
the  lens  gently  on  the  extemporised  rotary  axis  formed 
by  the  wedge  below  and  the  finger  above.  Now 
observe  if  the  image  of  the  candle  flame  stand  perfectly 
motionless,  or  whether,  as  will  most  likely  be  the  case, 
it  moves  across  the  card  with  every  rotation  of  the  lens. 
In  this  latter  case,  move  the  lens  a  little  farther  back- 
wards or  forwards  on  the  supporting  block  and  try 
again.  Do  this  until  that  position  is  found  at  which 
the  image  of  the  candle  remains  motionless  while  the 
lens  is  being  rotated  from  side  to  side,  and  then  put  a 
small  mark  on  the  tube,  which  ever  afterwards  will  in- 
dicate, with  the  degree  of  accuracy  practically  required, 


40  THE  MECHANICAL  CENTRE. 

the  optical  centre  of  the  combination,  whenever  em- 
ployed under  circumstances  in  which  the  position  of  the 
conjugates  assimilates  to  those  under  which  the  trial 
was  made. 

But  to  prove  that  the  centre  in  question  is  really 
only  that  of  these  conjugates  :  after  having  made  the 
mark  on  the  tube,  let  the  candle  be  brought  to  within 
six  feet  of  the  lens,  and  by  another  course  of  experiments 
let  its  centre  be  again  found,  and  it  will  be  seen  that 
it  now  differs  materially  in  position  from  that  of  the 
previous  trial.  The  new  centre  is  quite  right  for,  and 
under,  the  altered  conditions,  but  wrong  as  regards  all 
others. 

We  are  aware  of  some  gentlemen  who  are  so 
dexterous  in  examining  a  combination  for  its  optical 
centre  (we  are  now  using  the  term  under  a  kind  of 
protest)  that  they  will  take  it  up  and,  poising  it  between 
finger  and  thumb,  examine  the  stability  of  the  image 
on  the  wall  opposite  a  window  while  rotating  the  lens, 
and  in  this  way  will  in  less  than  half  a  minute  have 
acquired  more  knowledge  concerning  it  than  another 
would  in  some  days. 

The  Mechanical  Centre  not  the  Focal  Centre. — To  de- 
monstrate that  the  focal  centre  is  not  situated  in  the 
mechanical  centre,  let  us  take  the  case  of  a  combination 
of  the  cemented  doublet  class  so  commonly  used,  and 
let  us  further  assume  that  it  is  a  symmetrical  compound, 
that  is,  that  its  front  and  back  lenses  are  identical  in 
figure  and  focus.  Now  while  the  mechanical  centre  of 
such  a  combination  is  midway  between  the  lenses,  the 


MECHANICAL  VERSUS  FOCAL  CENTRES.      41 

focal  centre  may  be  anywhere  according  to  circumstances. 
This  will  be  readily  understood  from  the  following  con- 
siderations :  To  adduce  an  extreme  instance,  let  each 
lens  of  the  combination  be  twenty  laches  in  focus. 
These,  if  placed  so  close  together  as  to  be  merged  into 
one,  and  that  one  infinitesimally  thin,  would  have  a 
focus  of  ten  inches,  and  its  focal  centre  would  measure 
from  that  of  the  lens.  But  in  proportion  as  they  are 
separated  so  does  the  focal  centre  move  forward  in 
advance  of  the  mechanical  centre  ;  until  at  last  when 
our  hypothetic  mount  is  nineteen  inches  long  and  the 
back  lens  is  within  an  inch  of  the  ground  glass,  the  front 
lens  being  over  nineteen  inches  away,  where,  under 
such  conditions,  would  be  the  optical  or  focal  centre  for 
a  distant  object  ?  It  would  be  in  the  vicinity  of  the 
front  lens  and  many  inches  in  front  of  the  mechanical 
centre. 

The  focal  centre,  or  point  from  which  the  focus 
must  be  measured,  varies  therefore  according  to  the 
distance  of  the  object  in  front  or  its  anterior  or  major 
conjugate. 


CHAPTER  IX. 

SINGLE  ACHROMATIC  LENSES. 

Historical  Memoranda. — When  photography  was  youn^, 
various  devices  to  work  with  a  large  aperture,  and  at  the 
same  time  to  secure  sharp  definition,  were  had  recourse 
to.  It  had  been  early  found  that  simple  lenses  would 
not  answer  because  of  their  actinic  plane  of  representa- 
tion being  situated  nearer  to  the  lens  than  that  of  the 
visual  focus  ;  accordingly  the  single  lens  of  the  camera 
obscura  was  supplanted  by  the  achromatic  lens  of  the 
telescope,  the  surface  of  maximum  convexity  being 
placed  to  the  outside.  Owing  to  the  circumscribed  area 
of  definition,  the  lens  was  afterwards  reversed  as  regards 
position,  and  a  diaphragm  placed  in  front.  The  value 
of  Wollaston's  meniscus  lens  was  in  time  duly  recognised 
as  a  means  of  securing  an  extended  field  ;  and  we  find 
in  a  manual  by  Daguerre,  published  in  1839,  a  single 
meniscus  achromatic,  which  is  practically  that  manu- 
factured at  the  present  time,  subject  in  some  cases  to 
modifications  of  internal  curvature,  in  others  to  none. 

Diaphragms  Necessary  with  Landscape  Lenses.  —  Single 
achromatic  landscape  lenses  are  usually  either  of  plano- 
convex or  meniscus  form,  and  this  latter  is  the  more 


OF  LANDSCAPE  LENSES.  43 

pronounced  according  to  the  width  of  the  angle  of  view 
it  is  intended  to  include.  The  deeper  is  its  meniscus 
shape  the  smaller  must  be  the  stop,  and  the  nearer  must 
that  stop  be  to  the  lens.  If  a  single  lens  be  intended  to 
include  only  a  very  narrow  angle,  then  may  it  be  a 
crossed  one,  that  is,  both  sides  may  be  convex,  the  rela- 
tion of  the  radii  of  the  surfaces  not  being  arbitrary, 
although  approximately  as  one  to  six,  the  flatter  side 
being  outside  or  next  the  stop.  We  examined  an  old 
lens  of  this  form,  constructed  by  Goddard,  and  found,  as 
might  have  been  deduced  a  priori,  that  it  bore  a  dia- 
phragm unusually  large,  and  placed  at  a  considerable  dis- 
tance in  front,  but  that  its  covering  power  was  not  great. 

It  is  absolutely  necessary  that  to  ensure  the  best 
definition,  a  landscape  lens  must  have  a  diaphragm,  for 
in  this  respect  it  differs  from  combinations  which  may 
be  made  aplanatic.  The  reason  for  this  is  shown  in 
Fig.  n,  page  21.  But  this  lens  lends  itself  admirably 
to  those  who  desire  definition  of  a  low  order,  to  secure 
which,  all  that  is  necessary  is  to  use  it  either  without  any 
stop  at  all,  or  with  one  much  wider  than  the  fixed  dia- 
phragm which  the  optician  places  in  the  mount. 

Forms  of  Landscape  Lenses. —  In  Fig.  16  we 
give  the  earliest  form  of  landscape  lens  (that 
referred  to  in  Daguerre's  manual),  and,  as  stated, 
it  is  much  employed  at  the  present  time.  It 
consists  of  a  bi-convex  crown  cemented  to  a  bi- 
concave flint.  It  is  also  modified  by  being 
externally  a  plano-convex,  the  flint  glass  lens 
in  this  case  being  plano-concave.  FIG.  16. 


44  FORMS  OF  LANDSCAPE  LENSES. 

GruWs  Aplanatic.  —  The  first  departure  from  the 
above  form  was  made  in  1857,  by  Thomas 
Grubb,  who  reversed  the  relative  positions  of  the 
flint  and  crown,  as  shown  by  Fig.  17,  in  which 
the  lens,  meniscus  externally,  is  formed  of  a 
concavo-convex  flint  cemented  to  a  meniscus 
crown.  This  lens  was  found  to  bear  a  larger 
working  aperture  than  the  one  previously  men- 
tioned and  to  have  less  spherical  aberration, 
FIG.  17.  hence  his  selection  for  it  of  the  name  'Aplanatic/ 
Dallmeyer's  Wide -Angle  Landscape  Lens.  —  In  1865, 
J.  H.  Dallmeyer  introduced  a  modification  of  the  Grubb 
aplanatic,  shown  in  Fig.  18.  In  this  he  divided  the 
power  of  the  crown  glass  into  two,  one  placed  in 
front  and  the  other  behind  the  flint  glass.  In 
this  way,  by  sandwiching  the  flint  concave,  which 
was  soft,  between  the  two  hard  crown  -  glass 
menisci,  the  twofold  purpose  was  attained  of 
securing  the  softer  glass  from  abrasion  and  of 
effecting  better  correction,  for  he  was  not  con- 
fined to  making  both  the  crown  elements  of 
glass  of  similar  refractive  power.  A  subsequent 
FIG.  1 8.  modification  of  this  lens  has  been  made  by  T. 
R.  Dallmeyer,  who,  while  adhering  to  the  same  arrange- 
ment and  configuration  has,  by  the  adoption  of  other 
kinds  of  glass  than  that  employed  by  his  father,  adapted 
it  for  working  with  a  larger  aperture  than  was  formerly 
employed. 

An  American  Landscape  Lens. — A  lens   achromatised 
in  the  same  way  as  the  '  Globe '  has  been  employed  as 


LANDSCAPE  LENS  MOUNTING.  45 

a  single  landscape  combination.  As  shown  in  the 
figure  (Fig.  19)  it  consists  of  a  concavo-convex  flint 
and  a  meniscus  crown,  its  components  being  placed 
so  that  the  concave  surface  of  the  flint  is 
outside.  From  the  fact  that  this  class  of 
objective  is  very  seldom  to  be  met  with, 
the  inference  may  be  deduced  that  it  is 
inferior  in  general  utility  to  the  others  pre- 
viously described. 

When,  for  the  purpose  of  more  effective 
correction,  the  inner   or  contact  curves  of  a 
lens  are  of  short  radius,  the  thickness  may      FIG' I9< 
be   reduced    by   grinding   the   margin   of  the   concave 
surface    flat,    as    in    Fig.     20.       By    comparing    this 
with    the    previous    lens,   it    will    be    seen    to 
what  extent  a  gain  is  effected.     With   contact 
surfaces   of  the    same    radius    carried    out    to 
the    extreme  edge,   the    lens  would  be  abnor- 
mally thick.     The  flattened  portion  is  of  course 
protected     by    opaque    varnish    and    a    metal 
annulus.      The  light  being  transmitted  from  a 
diaphragm  rather  close  to  the  concave  surface, 
no  loss  is  sustained  by  the  smaller  dimensions  of 
the  less  dense  positive  element.     The  drawing 
is  one  of  a  wide-angle  combination  formed  of 
light  and  heavy  flint  rlass,  instead  of  crown  and  FIG.  20. 
flint. 

Landscape  Lens  Mounting. — By  whichever  method  a 
single  landscape  lens  is  corrected,  it  must  be  mounted 
with  the  stop  next  to  its  flatter  side,  as  indicated  in 


46         GODDARUS  DOUBLE  PER  ISC  OP  1C  LENS. 

Fig.   21,    which    represents    one   of  tie    best   forms   of 
wide-angle  lenses. 


FIG.   21. 

Non-distorting  Landscape  Lenses — It  was  long  held  to 
be  impossible  for  a  lens  of  the  single  genus,  having  a 
diaphragm  in  front,  to  give  a  rectilinear  picture  ;  but,  in 
1859,  tne  late  James  T.  Goddard,  under  the  title  of  the 
double  periscopic  lens,  made  several  which,  however, 
never  came  into  general  use.  This  lens,  externally,  was 
a  double  convex,  and  all  the  parts  were  cemented 
together  in  such  a  manner  as  to  afford  no  clue  as  to  its 
internal  figuration.  One  of  these  which  soon  afterwards 
fell  into  our  hands  was  dissected  by  placing  it  in  warm 
v.ater  and  melting  the  balsam.  It  was  then  found  that 
the  interior  portion  of  the  lens  was  a  double  convex 
crown  lens  cemented  to  a  double  concave  flint,  the  two 
neutralising  each  other  in  respect  to  magnifying  power, 


DALLMEYER'S  RECTILINEAR  LENS.  47 

and  that  the  back  element  was  a  meniscus  of  rather 
deep  curvature  and  formed  of  crown  glass.  This  lens 
was  mounted  with  a  stop  in  front  of  it.  It  will  be  seen 
from  this  description  that  there  was  a  considerable  air 
space  between  the  back  lens  and  the  cemented  portion. 

Dallmeyer's  Rectilinear  Landscape  Lens. — In  1887,  T.  R. 
Dallmeyer  introduced  a  lens  which,  as  implied  by  its 
name,  gave  freedom  from  distortion.  In  it  he  makes 
a  species  of  compromise  between  the  purely  landscape 
lens  and  the  rectilinear  combination,  and  he  effects  this 
by  displacing  one  of  the  crown  elements  of  his  triple 
landscape  objective  and  transferring  it  in  a  reversed 
position  to  the  opposite  side  of  its  confreres.  It  is  known 
to  those  who  have  studied  this  landscape  lens,  that  when 
one  of  its  crown  elements  is  removed,  a  flint  negative 
and  crown  positive  still  remain,  in  which  there  is  little 
or  no  power  either  for  magnifying  or  diminishing.  It 
is,  however,  a  powerful  corrector  of  the  aberration  of  the 
third  element.  In  Dallmeyer's  new  lens,  he  makes  as  it 
were  a  separate  element  of  these  two  glasses  and  turns 
the  convex  surface  towards  the  diaphragm,  while  im- 
mediately behind,  he  places  the  crown  meniscus,  by 
which  the  focus  is  determined.  As  the  concave  surfaces 
of  both  are  next  each  other,  there  is  thus  an  air-space 
left  between  them.  Although  this  lens  possesses  some 
general  features  in  common  with  Goddard,  it  cannot,  in 
any  sense,  be  considered  to  be  a  copy  of  his,  because, 
in  the  first  place,  the  only  Goddard  lens,  the  existence 
of  which  we  can  learn,  has  never  been  out  of  our  own 
possession  since  it  was  made,  and  secondly,  that  Dall- 


48  ADVANTAGES  OF  SINGLE  LENSES. 

meyer's  lens  has  a  totally  different  internal  construction 
which,  we  may  add,  gives  it  great  advantages  in  covering 
power  with  better  definition. 

Advantages  of  Single  Lenses.— Single  lenses  possess  an 
advantage  over  double  ones  in  respect  to  the  pluck  and 
vigour  which  they  yield,  a  landscape  being  the  subject. 
Being  formerly  constructed  with  only  small  diaphragms, 
they  were  necessarily  slow  in  action,  but  this  drawback 
has  now  been  surmounted,  some  of  the  better  class 
working  with  an  aperture  of  even  f-S.  With  an  aperture 
of  this  width  it  need  scarcely  be  said  portraits  can  be 
easily  obtained  ;  indeed,  for  portraiture,  there  is  a  special 
charm  in  this  class  of  lens,  on  account  of  the  delicate 
softness  which  can  be  obtained  when  working  with  an 
abnormally  great  angular  aperture.  Of  course,  to  cover 
an  extended  field  sharply,  a  small  stop  must  be  employed, 
and  as  we  have  shown  in  anothei  chapter  the  stop  in 
the  lens  must  be  so  placed  as  not  to  give  a  flare-spot. 


CHAPTER  X. 

DISTORTION  :    ITS   NATURE  AND   CURE. 

THERE  are  several  kinds  of  distortion  capable  of 
being  produced  in  photography.  These  include  that  of 
'  violent  perspective/  caused  by  placing  the  camera  too 
close  to  the  object  to  be  taken,  whether  portrait,  land- 
scape, or  building. 

Point  of  Sight  too  Near, — This  kind  of  distortion  is 
seen  in  portraits  in  which  the  feet  or  hands  are  pro- 
jected forward  and  represented  on  a  scale  of  magnitude 
greatly  surpassing  that  of  the  figure  itself.  The  remedy 
for  this  consists  in  removing  the  camera  to  such  a 
distance  from  the  object  as  to  reduce  all  the  parts 
practically  to  the  same  uniform  scale  of  representation. 
The  employment  of  lenses  of  too  short  a  focus  has  much 
to  answer  for  in  the  production  of  this  distortion  of  per- 
spective. In  these  cases  the  perspective  itself  is  not 
necessarily  false — it  is  only  violent ;  but  it  conveys  an 
erroneous  idea.  In  landscapes  it  causes  insignificant 
ponds  in  the  foreground  to  become  considerable  lakes, 
and  tiny  rivulets  to  assume  the  magnitude  of  rivers. 

Distortion  of  Convergence. — There  is  also  a  very  common 
form  of  distortion  exemplified  in  the  contraction  of  the 

E 


50  DISTORTION. 

scale  of  representation  towards  one  margin  of  the 
picture.  It  is  usually  seen  in  photographs  of  buildings, 
and  gives  them  an  appearance  as  if  they  were  leaning 
towards  an  imaginary  central  line  for  support.  This 
may  be  termed  the  '  distortion  of  convergence.'  It 
arises  from  no  fault  in  the  lens,  but  from  the  want  of 
care  or  of  knowledge  in  the  photographer,  who,  desirous 
of  including  the  whole  of  a  building  in  his  plate,  has 
tilted  his  camera  slightly  upwards  without  utilising  its 
swing-back  to  bring  the  plate  into  a  perfectly  vertical 
state  ;  for  one  of  the  rigid  conditions  which  govern  the 
taking  of  a  building  properly  is  this — that,  no  matter 
how  much  the  lens  or  camera  may  be  pointed  upwards, 
the  plate  itself  must  be  perfectly  vertical. 

Curvature  of  Straight  Lines, — There  are  other  kinds  of 
distortion,  but  none  that  is  justly  chargeable  to  the  lens 
save  that  very  important  one  known  as  '  curvilinear 
distortion,'  the  chief  characteristic  of  which  is  the  curva- 
ture imparted  in  the  photograph  to  lines  that  are  quite 
straight  in  the  original.  This  defect  is  produced  solely 
by  the  lens,  and  no  skill  in  the  photographer  can  obviate 
it  so  long  as  a  lens  of  that  description  is  employed. 

Every  ordinary  objective  having  its  stop  between  the 
lens  itself  and  the  subject  to  be  reproduced  will  give 
distortion.  No  matter  how  perfect  a  landscape  lens 
may  bo — how  superb  its  definition  or  penetrative  its 
range  ;  though  it  may  reproduce  the  finest  line  of  the 
finest  engraving  with  all  the  crispness  of  the  original 
and  delineate  the  very  structure  of  the  stones  of  which 
an  edifice  is  formed,  yet  it  will  not  be  either  a  copying 


btSTORTION.  51 

or  an  architectural  lens.  These  demand  not  only  all  the 
qualities  mentioned  but  also  something  more,  namely, 
absolute  rectilinearity  in  projection.  The  appearance 
presented  in  a  photograph  taken  with  a  landscape  lens 
in  which  a  building  is  made  to  cover  nearly  the  entire 
plate  suggests  the  form  of  a  huge,  wide  barrel,  owing  to 
all  the  straight  lines  curving  inwards  towards  the  centre. 
In  such  a  picture  only  two  lines  are  quite  straight — 
those  which  pass  vertically  and  horizontally  through  the 
centre  of  the  photograph.  No  matter  how  much  the 
lens  distorts,  these  centre  lines  are  always  straight ;  but, 
in  proportion  as  we  proceed  towards  the  margin,  we 
find  them  becoming  more  and  more  curved.  As  this 
defect  is  not  much  noticed  near  the  centre,  it  follows 
that  one  may  take  a  view  of  a  house  or  church  without 
any  apparent  distortion  so  long  as  its  position  is  kept 
near  the  centre  of  the  plate  ;  but  for  copying  a  map, 
chart,  or  any  kind  of  engraving  in  which  accuracy  is  a 
sine  qua  non,  it  is  altogether  unsuitable. 

Cause  of  Distortion.  —  Having  indicated  the  nature, 
we  shall  now  consider  the  cause  of  distortion.  Bearing 
in  mind  what  has  been  said  in  a  previous  chapter  con- 
cerning the  possibility  of  taking  a  photograph  without 
any  lens  whatever,  merely  by  transmitting  the  rays  from 
the  object  through  a  pinhole  aperture  in  front  of  the 
camera,  we  remark  that  any  copy  of  a  picture  or  repre- 
sentation of  a  natural  object  made  by  such  pinhole  will 
be  quite  rectilinear,  for  with  such  an  arrangement  the 
light  passes  in  straight  lines  without  refraction.  Let 
us  consider  in  what  manner  these  rays  are  influenced 


52  BARREL  DISTORTION. 

by  a  lens  so  as  to  disturb  rectilinearity  of  projection. 
It  has  been  shown  that  the  margin  of  a  lens  refracts 
in  a  greater  degree  than  its  centre ;  that,  in  short,  one 
of  a  set  of  parallel  incident  rays  is  transmitted  through 
the  centre  of  a  lens  without  undergoing  any  refraction 
at  all,  and  that  in  proportion  as  the  point  of  trans- 
mission is  near  the  margin  or  towards  the  centre  so 
does  the  ray  thus  transmitted  become  refracted  in  a 
greater  or  lesser  degree.  All  rays  which  come  from 
a  perfectly  square  map  or  building  are  quite  right 
while  passing  through  the  diaphragm  and  up  to  their 
passage  through  the  lens,  when  they  are  brought  under 
the  influence  of  its  dimensions,  with  the  result  already 
described.  The  square  original  becomes  barrel-shaped 
in  the  photograph,  as  shown  in  Fig.  22,  in  which  the 


FIG.  22. 


curvature  is,  in  order  to  show  the  principle,  more  pro- 
nounced than  it  would  be  with  an  ordinary  photographic 
lens,  because  as  the  margin  of  a  picture  is  taken  with 
the  margin  of  the  lens,  that  margin,  owing  to  its  superior 
refractive  power  'condenses'  the  rays  into  a  smaller 
space  or  bends  them  towards  its  axis,  thus  causing  a 
given  portion  of  the  original  to  occupy  a  smaller  space 
near  the  margin  than  it  would  do  at  the  centre  of  the 
photograph. 


PINCUSHION  DISTORTION.  53 

This  is  the  invariable  result  of  employing  a  lens, 
such  as  a  single  landscape  objective,  in  which  the  stop 
is  in  front.  What,  it  might  be  inquired,  would  be  the 
result  if  the  objective  were  turned  round  so  as  to  allow 
the  light  to  pass  through  the  lens  before  it  reached  the 
diaphragm  ?  Simply  this  :  that  the  nature  of  the  dis- 
tortion would  be  changed.  There  would  be  an  expansion 
of  the  scale  at  the  margin  instead  of  a  reduction  as  in 
the  former  case,  and  the  resulting  picture  would  have  its 
marginal  lines  bent  outwards  like  a  pincushion,  as  shown 
in  Fig.  23,  from  which  has  arisen  the  term  '  pincushion  ' 


FIG.  23. 

distortion,    now   recognised    as    the    antithesis   to   the 
'  barrel '  distortion  already  described. 

The  nature  and  cause  of  distortion  having  been 
explained  with  all  the  fulness  required  in  a  popular 
disquisition  like  the  present,  we  now  come  to  speak 
of  the  various  methods  adopted  for  effecting  its  cure. 
During  a  long  period  it  was  the  earnest  aspiration  of 
both  opticians  and  photographers  to  obtain  a  lens  which 
would  give  freedom  from  distortion,  and  here  in  this 
connexion  we  would  record  one  of  the  most  remarkable 
things  in  optical  history.  While  opticians  were  straining 
to  devise  a  lens  which  should  give  freedom  from  dis- 
tortion, it  was  already  in  their  hands,  although  seemingly 


54 


DISTORTION. 


they  knew  it  not.  So  long  ago  as  1844,  Geo.  S.  Cundell 
had  published  in  the  October  number  of  the  Philosophical 
Magazine  of  that  year  a  symmetrical  combination  of 
lenses  of  form  similar  to  the  rapid  rectilinear  class  of 
the  present  period,  being  meniscus  lenses,  although  un- 
corrected,  with  a  diaphragm  midway  between  the  lenses. 
How  opticians  failed  to  recognise  in  this  combination 
the  panacea  for  the  evils  of  distortion  is  truly  surprising. 
Cundell's  lens,  which  was  apparently  entirely  lost  sight  of 
by  opticians,  having  been  achromatised,  is  now  the  lens 
of  the  day.  And  here,  as  a  sequel  to  the  immediately 
foregoing  illustrations  of  the  two  kinds  of  distortion 
produced  by  single  lenses,  we  show  in  what  manner  the 
combination  referred  to  cures  distortion  (Fig.  24).  It 


FIG.  24. 

eventually  began  to  dawn  upon  those  people  who  gave 
thought  to  the  matter,  that  if  a  diaphragm  placed  in 
front  of  a  lens  gave  barrel-shaped  distortion,  and  a 
diaphragm  behind  the  lens  gave  distortion  of  the  opposite 
or  pincushion  character,  a  diaphragm  placed  midway 
between  two  lenses  would  give  no  distortion  at  all. 
And  so  it  was. 

But  at  a  date  three  years  anterior  to  the  publication 
of  the  Cundell  lens,  the  late  Andrew  Ross  had  con- 
structed for  Henry  Collen  a  portrait-objective,  composed 


ORTHOSCOPIC  LENS.  55 

of  two  plano-convex  achromatic  lenses  with  a  stop 
midway  between,  and  during  the  subsequent  years  of 
the  life  of  this  optician  it  does  not  appear  to  have 
occurred  to  him  (as  it  did  twenty-three  years  afterwards 
to  his  son,  Thomas  Ross),  that  this  form  satisfied  the 
conditions  of  freedom  from  distortion,  and  that  it  was 
only  necessary  to  make  its  components  of  a  meniscus, 
instead  of  a  plano-convex  form,  to  flatten  its  originally 
round  field. 

Contemporaneous  with  Andrew  Ross  was  Thomas 
Davidson,  an  Edinburgh  optician,  who  produced  some 
symmetrical  achromatic  lenses  which  were  quite  free 
from  distortion,  which  will  be  described  in  another 
chapter. 

The  Condition  for  ensuring  N  on -distortion. — The  con- 
dition that  must  be  fulfilled  by  any  combination  of 
lenses  in  order  that  there  shall  be  no  distortion,  is  this — 
each  ray  that  enters  the  combination  must  emerge  from 
it  in  a  direction  parallel  to  that  of  its  entry.  If  the 
immergent  ray  makes  a  certain  angle  with  the  axis  of 
the  lens,  the  emergent  one  must  make  a  similar  angle. 

Advent  of  the  Orthoscopic  Lens. — The  cry  for  non- 
distorting  lenses  was  at  its  loudest,  and  all  those  just 
described  had  been  forgotten  or  ignored  when,  in  the 
beginning  of  1857,  Voigtlander  introduced  his  orthoscopic 
lens,  which  was  constructed  on  a  formula  supplied  by 
Petzval.  The  orthoscopic  lens  became  the  ( rage.' 

Several  claims  were  put  forward  on  behalf  of  this 
lens,  and  Thomas  Sutton,  editor  of  Photographic  Notes, 
who  was  a  facile  writer  on  mathematical  optics,  descanted 


56  ORTHOSCOP1C  LENS. 

in  the  most  rapturous  terms  upon  its  numerous  virtues 
— its  entire  freedom  from  distortion,  its  flatness  of  field, 
equality  of  illumination,  perfection  of  focus,  and  freedom 
from  spherical  aberration.  The  orthoscopic  lens  was 
to  prove  the  panacea  for  every  ill.  It  was  everywhere 
spoken  of;  and,  having  become  the  fashion,  there  were 
some  weak-minded  photographers  who  scarcely  dared 
venture  to  assert  that  any  specially  fine  picture  they 
had  taken  had  perchance  been  obtained  by  the  aid  of 
the  old-fashioned  landscape  lens.  But  fashions  change 
in  lenses  as  in  other  things,  and  subsequently  it  was 
found  that  the  once-idolised  orthoscopic  lens  did  not 
possess  freedom  from  distortion,  that  its  field  was  not 
flat ;  that  in  equality  of  illumination  and  perfection  of 
focus  it  was  not  a  whit  better  than  the  old  landscape 
lens.  And  so  the  orthoscopic  lens  was  deposed  from 
its  position  of  reigning  favourite  and  well-nigh  lost  sight 
of.  What  is  to  be  regretted  is  that  the  foolish  claim 
implied  in  its  name  was  ever  put  forth,  because  any 
careful  observer  could  upon  close  examination  have  dis- 
covered that  it  did  distort,  although  from  the  position 
of  the  diaphragm  the  distortion  was  of  an  opposite 
character  to  that  previously  experienced. 

In  a  subsequent  description  of  lenses  we  shall  not 
be  disposed  to  treat  the  orthoscopic  objective  as  defunct, 
because,  when  the  absurd  claim  made  for  it  upon  its 
introduction  has  been  set  aside,  it  possesses  special 
features  and  virtues  of  a  marked  order  which  may 
eventually  secure  for  this  instrument  a  recognition  and 
patronage,  doubtless,  greatly  exceeding  that  first  accorded 


ORTHOSCOPIC  LENS.  57 

to  it.  In  saying  this  we  are  fortified  by  the  expression 
of  opinion  of  one  of  the  ablest  mathematical  and  practical 
opticians  of  the  present  time,  to  the  effect  that  in  the 
orthoscopic  lens,  when  subjected  to  certain  modifications 
of  structure,  may  yet,  possibly,  be  found  one  of  the 
'lenses  of  the  future/ 

A  word  in  passing  concerning  the  name  'orthoscopic' 
or  'orthographic' — signifying  respectively  correct  seeing 
or  correct  delineating.  It  is  much  more  applicable  to 
the  doublet  lenses  of  the  present  time,  which  are  really 
rectilinear  (a  term  having  an  analogous  meaning),  than 
to  that  form  about  which  we  have  been  writing.  We  are 
rather  pleased  than  otherwise  to  find  that  an  American 
optician  has  lately  re- adapted  the  name  to  a  lens  of  the 
rectilinear  class  which  he  makes. 


CHAPTER  XI. 

NON-DISTORTING   LENSES. 

THE  nature  of  distortion  having  been  fully  treated 
in  a  previous  chapter,  we  now  enter  upon  a  consideration 
of  the  various  lenses  which  have  been  constructed  with 
a  special  view  to  freedom  from  this  error. 

The  Ortkoscopic  Lens. — We  have  already  alluded  to 
this  as  having  been  the  first  objective  presented  to  the 
public  with  a  direct  claim  to  correctness  in  linear  pro- 
jection, although  such  claim  was  subsequently  abandoned 
The  following  is  a  description  of  it :  It  consists  of  a 
plano-convex  or  nearly  flat  achromatic  meniscus,  similar 
to  the  front  lens  of  the  Petzval  portrait  combination, 
and  used  in  the  same  position.  At  a  very  short  distance 
behind  this  is  placed  an  achromatic  lens,  somewhat 
smaller  in  diameter  and  concave  as  a  whole  ;  that  is 
to  say,  it  diminishes  instead  of  magnifies.  Although 
Voigtlander,  the  first  maker  of  this  objective,  con- 
structed it  with  a  smaller  back  than  front — these  being 
in  the  ratio  of  2\  inches  to  ij  inches — yet  did  some 
other  makers  form  both  front  and  back  of  equal 
diameters.  The  first  element  of  this  back  lens  is 
formed  of  a  bi-concave  crown  glass,  the  radii  of  the 


ORTHOSCOPIC  LENS. 


59 


surfaces  being  unequal ;  the  second  element  is  a  flint 
glass  meniscus,  and  this  back  lens  both  materially 
lengthens  the  focus  of  the  front  one  and  flattens  the 
field,  at  the  same  time  correcting  the  oblique  pencils. 


FIG.  25. 

This  it  does  in  right  of  the  fact  that  an  oblique  pencil 
falling  upon  a  concave  lens  is  powerfully  affected  by  it, 
being  considerably  lengthened  in  focus.  Indeed,  with 
a  combination  of  this  nature,  it  is  easy  to  have  a  back 
lens  of  such  a  kind  so  adjusted  to  the  front  as  to  cause 
the  oblique  pencils  to  be  so  much  longer  than  the 
central  ones,  that  the  field  shall  be  not  merely  flat 
but  bellied  in  the  opposite  direction  from  that  in 
which  photographers  are  accustomed  to  see  it. 

Much  was  said  concerning  the  equality  of  illumi- 
nation possessed  by  the  orthoscopic  lens  at  the  time 
it  was  introduced,  and  much  was  written,  even  by  talented 


60  ORTHOSCOPIC  LENS. 

opticians  (e.g.  Andrew  Ross)  to  prove  its  superiority  in 
this  respect  over  the  single  achromatic  landscape  lens  ; 
but  although  the  author  possesses  some  of  the  best 
specimens  of  this  kind  of  lens  that  have  been  made, 
he  has  quite  failed  to  discover  their  superiority  in  this 
respect  over  ordinary  lenses. 

The  Causes  of  Unequal  Illumination. — These  are,  first, 
the  fact  that  a  pencil  transmitted  obliquely  through  a 
circular  aperture  (the  diaphragm)  is  smaller  than  one 
transmitted  directly  or  centrally  through  the  same 
aperture  ;  and,  secondly,  that  the  pencil  thus  transmitted 
obliquely  is  not  merely  smaller  in  diameter,  but  it  has 
farther  to  travel  and  more  work  to  accomplish.  This 
is  the  case  with  every  lens  by  which  an  oblique  pencil 
is  transmitted  through  a  circular  aperture. 

Position  of  the  Diaphragm  in  the  Orthoscopic  Lens. — 
The  orthoscopic  lens  was  somewhat  extensively  con- 
structed by  opticians  after  its  introduction,  and  was 
sold  under  a  variety  of  names.  It  is  worthy  of  being 
noted  that  while  Voigtlander  placed  the  diaphragm 
behind  the  back  lens,  Ross  inserted  it  between  the 
front  and  back,  while  Goddard  placed  it  outside  of 
the  front  lens.  It  is  difficult  to  surmise  why  he  did 
so,  unless  on  the  supposition  that  realising  the  optical  or 
focal  centre  was  quite  outside  of  the  front  lens,  he  sought 
to  minimise  distortion  by  having  the  stop  as  near  to 
that  centre  as  possible. 

A  Unique  Property  in  the  Orthoscopic  Lens. — A  special 
virtue  possessed  by  the  orthoscopic  lens,  and  by  no  other, 
consists  in  the  ability  of  obtaining  with  it  larger  sized 


bOUBLE  PERI  SCOP  1C.  6l 

images  in  the  negative  with  a  given  extension  of  camera 
than  can  be  obtained  by  any  other  lens  extant.  The 
size  of  the  image  depends  upon  the  focus  of  the  lens 
by  which  it  has  been  taken.  The  focus  of  a  lens  is 
measured  from  and  determined  by  the  position  of  its 
focal  centre ;  and  while  this  in  a  single  landscape  lens 
is  rather  nearer  to  the  ground  glass  than  the  lens  itself, 
it  is  in  the  orthoscopic  combination,  as  just  stated,  out- 
side of  the  lens  entirely,  so  that,  with  a  given  length 
of  camera,  a  much  larger  image  of  an  object  can  be 
obtained  by  the  orthoscopic  lens  than  by  any  other. 
This  is  a  property  of  great  value. 

Goddard's  Double  Periscopic. — The  name  of  James  T. 
Goddard  occupies  an  honourable  position  among  those 
opticians  who  have  directed  their  efforts  to  the  intro- 
duction of  lenses  different  from  those  which  previously 
existed,  in  order  to  eliminate  with  more  or  less  success 
their  inherent  faults. 

Among  lenses  introduced  by  Goddard  was  a  recti- 
linear landscape  objective  which  he  designated  his 
'double  periscopic'  lens.  This  was  in  January,  1859. 
Externally  this  lens  was  of  double  convex  form  ;  but 
there  was  an  air-space  inside,  and  it  was  constructed  as 
follows :  The  front  surface  was  that  of  a  biconvex 
crown,  cemented  to  a  biconcave  flint,  these  two  forming 
a  meniscus  combination  without  any  positive  or  mag- 
nifying power.  Cemented  by  its  margin  to  this  was 
a  meniscus  of  crown  glass,  the  residuum  of  the  over- 
correction  for  colour  of  the  front  portion  effecting  the 
correction  of  this  meniscus.  Used  with  a  diaphragm 


62  GODDARDS  TRIPLE  LENS. 

in  front,  this  objective  was  free  from  distortion.  Its 
marginal  definition,  however,  is  inferior  to  another  since 
constructed,  on  the  same  general  principle,  by  T.  R. 
Dallmeyer,  and  as  an  independent  invention,  he  not  then 
being  aware  of  Goddard's  lens.  See  pages  46  and  47. 

Goddard's  Triple  Lens. — About  the  same  time  as  the 
'  periscopic '  was  introduced,  Goddard  constructed  a 
triple  objective,  the  front  of  which  was  an  ordinary 
shallow  achromatic  meniscus,  the  centre  lens  being  a 
biconcave  and  the  back  a  deep  meniscus.  The  centre 
lens  was  smaller  than  the  others,  but  neither  it  nor  the 
back  lens  was  achromatised.  The  front  achromatic  and 
the  back  meniscus  were  of  similar  f~ci,  the  power  of  the 
intermediate  concave  being  such  as  to  neutralise  the 
magnifying  power  of  either  of  them. 

Goddard's   Combination    Landscape    Lens.  —  This    lens, 
introduced  at  the  same  period  as  the  two  preceding,  has 
an  achromatised,  front  of  meniscus  form.     The  anterior 
of    the    back     combination     is    a 
biconcave     of     crown     glass,    the 
posterior    being    a    meniscus    also 
of    crown.       The     curvatures     of 
these   two    are    such    as    to    prove 
that   they  possess    no    magnifying  n 


power.       The    distance    apart    of  FIGi  26 

the  front  and   back   combinations 

is  not  an  arbitrary  one,  but  may  be  altered  to  suit 
the  circumstances  of  each  case.  When  separated  some- 
what, the  marginal  definition  is  much  improved  and 
the  field  flatter  than  when  they  are  brought  close 


SUTTOWS  SYMMETRICAL  TRIPLET.  63 

together.  The  focus  of  the  combination  is,  therefore,  that 
of  the  achromatic  meniscus ;  and  Goddard's  idea  was 
to  supply  a  variety  of  these  mounted  in  separate  cells, 
so  that  the  photographer  having  a  mount  containing 
one  correcting  back  lens — for  he  (Goddard)  preferred 
giving  the  achromatic  lens  the  anterior  position  in  the 
mount — could  make  use  of  several  achromatic  lenses  of 
any  required  focus. 

The  advantages  claimed  for  the  combination  just 
described  were  freedom  from  distortion,  flatness  of  field, 
and  the  ability  for  adapting  a  number  of  front  lenses, 
each  varying  in  focus  from  another,  to  the  combination. 
On  referring  to  some  notes  made  when  inspecting 
Goddard's  work-book  after  his  death,  we  find  that  he 
frequently  departed  from  the  form  shown  in  the  above 
diagram,  occasionally,  inter  alia,  adopting  the  plano- 
concave instead  of  the  double-concave  form  for  the 
crown,  and  sometimes  separating  the  two  crown  glass 
lenses  to  a  considerable  extent. 

Button's  Symmetrical  Triplet. — In  1860  Thomas  Sutton 
introduced  a  lens  under  this  name.  It  was  composed  of 
two  achromatised  plano-convex  lenses  of  similar  foci, 
mounted  at  either  end  of  a  tube,  with  a  simple  bi- 
concave lens  in  the  middle.  The  power  of  this  latter 
was  such  as  to  neutralise  either  of  the  outer  two,  but 
only  few  of  them  were  made. 

Dallmeyer's  Triple  Achromatic. — A  special  form  of 
triple  lens  which  secured  a  great  degree  of  favour 
among  photographers  is  shown  in  Fig.  27,  which  re- 
presents the  triple  achromatic  combination  of  J.  H. 


64 


DALLMEYEKS  TRIPLE  ACHROMATIC. 


Dallmeycr.  The  flatter  surfaces  of  the  front  and  back 
lenses  are  slightly  concave,  differing  to  this  extent  from 
a  triple  lens  subsequently  introduced  by  Thomas  Ross, 


FIG.  27. 

in  which  these  surfaces  were  quite  flat.  The  triple 
just  shown,  together  with  the  others  mentioned  in  this 
chapter,  is  quite  free  from  distortion. 

By  increasing  the  diameter  of  the  middle  lens,  Dall- 
meyer  subsequently  constructed  a  triple  objective  having 
an  angular  aperture  sufficiently  great  to  enable  it  to  be 
employed  for  groups  and  portraiture. 


CHAPTER  XII. 

WIDE-ANGLE  NON-DISTORTING  LENSES. 

Defining  the  Term. — It  is  difficult  to  draw  a  sharp 
line  of  demarcation  between  narrow  angle,  medium 
angle,  wide  angle,  and  panorama,  seeing  that  they  im- 
perceptibly merge  into  each  other.  We  may,  however, 
hazard  the  opinion  that  an  included  angle  of  subject 
up  to  25°  fittingly  comes  under  the  first  of  these  terms; 
one  up  to  45°  being  medium  ;  whilst  a  lens  that  includes 
more  than  a  view  of  which  the  base  equals  the  focus, 
may  be  relegated  to  those  of  wide  angle.  But  many 
wide-angle  lenses  include  an  angle  of  90°  on  the  base 
line,  and  hence  the  application  of  the  distinguishing 
terms  can  at  best  be  only  approximative. 

Button's  Panoramic  Lens. — This  lens,  which  doubtless 
covered  a  wider  angle  than  any  previously  introduced, 
must  at  present  be  spoken  of  in  the  past  tense,  none  of 
them  being  now  made.  It  was  composed  of  two  thick 
concentric  shells  of  flint  glass,  all  the  surfaces  being 
measured  from  a  common  centre.  It  was  in  effect  a 
sphere  of  glass,  the  space  in  the  middle  being  filled 
with  water.  It  was  achromatic,  and  the  spherical 

F 


66  THE  GLOBE  LENS. 

aberration  was  sufficiently  corrected  to  admit  of  its 
taking  pictorially  sharp  photographs.  An  ingenious 
'  butterfly  '  diaphragm  was  de- 
vised, by  which  the  extreme 
side  of  the  image  was  illumi- 
nated with  the  same  intensity 
as  the  centre.  But  at  the  time 
when  it  was  introduced,  it  was 
imperative  that  the  image  had 
to  be  received  on  a  curved  or 
cylindrical  plate,  the  printing- 
frame  and  other  fittings  being 

also    curved,    and    this    led    to    its    manufacture    being 
discontinued. 

The  objection  formerly  existing  need  not  now  pre- 
vail, for  sensitive  celluloid  or  other  flexible  plates  can 
be  placed  in  flexible  or  roller  slides,  and,  by  suitable 
curved  guides  at  the  back  of  the  camera,  can  be  tem- 
porarily bent  in  the  cylindrical  form  and  afterwards 
flattened  out.  The  author  has  ascertained  from  actual 
experiment  the  practicability  of  the  suggestion  here 
made,  having  taken  by  one  of  these  lenses  and  on  a 
celluloid  film  a  panoramic  view  embracing  an  angle 
of  125°  in  the  fractional  part  of  a  second — an  angle 
exceeding  by  5°  that  which  the  lens  was  originally 
computed  to  cover. 

The  Globe  Lens. — This  is  an  American  production, 
so  named  because  its  external  surfaces,  like  those  of 
Button's,  formed  portions  of  a  sphere  relative  to  each 
other.  It  is  composed  of  a  symmetrical  pair  of  deep 


MORRISON'S  WIDE-ANGLE  LENS. 


FIG.  29. 


to    exorcise    the 


meniscus  lenses,  achromatised  by  the  union  of  a  con- 
cavo-convex flint   cemented  to  a  meniscus  crown,  the 

latter  being  placed  outside,  as  shown  in  the  cut.     The 

diaphragm  is  in  the  middle   of 

the    objective.       Some    of    the 

Globe   lenses   gave  a  flare-spot 

or  ghost    in    the   centre   of  the 

picture,  and  it  does  not  seem  to 

have  occurred  to  C.  C.  Harrison 

of  New   York,     the    maker,    to 

have  set  aside  the  *  globe  '  idea  in 

their  construction,  and  mounted 

them    a    little    closer    together. 

This    slight    modification    we    found 

ghost  entirely. 

The  '  Globe  '  was  subjected  to  modifications  by  other 

makers  of  the  period  and  country,  but  the  same  general 

feature  pervaded  them  all. 

Morrison's  Wide-angle  Lens. —  Richard  Morrison,  on 
the  death  of  Harrison,  in  whose  em- 
ployment he  had  long  been,  conceived 
an  idea  that  some  advantage,  especially 
in  construction,  would  accrue  by  slightly 
over-correcting  one  of  the  lenses  of  the 
'globe/  and  supplying  the  place  of  the 
other  with  a  simple  crown-glass  me- 
niscus This  idea  was  not  quite  original, 
for,  so  long  ago  at  1857,  it  was  placed 

upon  record  that    F.  H.  Wenham  had  had    a   lens    (a 

narrow-angle  one,  however)  constructed  for  him  in  which 


FIG.  30. 


(l 


68  DOUBLETS. 

the  front  was  a  plano-convex  lens  of  crown  glass,  the 
back  lens  being  an  over -corrected  achromatic,  also  of 
plano-convex  form.  We  carefully  examined  a  lens 
received  direct  from  Mr.  Morrison,  and  found  that 
although  the  front  lens  was  not  really  over-corrected 
for  colour,  yet  that  the  addition  of  the  crown-glass  back 
did  not  appreciably  affect  its  working  to  visual  focus. 
Lenses  of  deep  meniscus  form  possess  a  wonderful  de- 
gree of  elasticity  as  regards  focus. 

Steinheil's  Periskop.  —  Dr.  A.  Steinheil,  about  a 
quarter  of  a  century  ago,  introduced 
a  symmetrical  doublet  constructed  ex- 
pressly for  including  a  wide  angle. 
It  was  of  simple  form,  being  composed, 
as  shown  in  the  figure,  of  two  simple 
or  uncorrected  lenses  formed  of  crown 
glass.  It  embraced  a  very  wide  angle 
of  view,  but  having  an  exceedingly 

FIG.  31. 

small  diaphragm  it  worked  slowly  in 
those  days  of  wet  collodion,  and,  besides,  the  visual 
and  chemical  foci  were  not  coincident. 

Zentmayei's  Lens. — Josef  Zentmayer,  of  Philadelphia, 
improved  upon  the  Steinheil  periskop  by  making  it  un- 
symmetrical,  the  back  lens  being  of  shorter  focus  than 
the  front,  and  the  diaphragm  being  placed  nearer  the 
back,  in  the  ratio  of  the  foci  of  each  component. 

The  Doublets  of  Grubb  and  Ross. — We  have  to  link 
these  optical  productions  together,  because  both  were 
introduced  at  the  same  period.  It  became  more  in- 
timately associated  with  the  name  of  Ross,  as  Thomas 


GENESIS  OF  THE  ROSS  DOUBLET. 


69 


Ross  manufactured  it  in  three  different  degrees  of  in- 
cluded angle,  while  the  professional  engagements  of 
Thomas  Grubb,  as  chief  engineer  to  the  Bank  of  Ire- 
land, prevented  him  at  that  time  from  bestowing  much 
attention  upon  it.  A  good  idea  of  its  nature  will  be 
ascertained  from  the  figure,  in  which 
A  is  the  front  lens  and  B  the  back 
lens.  By  rendering  the  components 
of  a  more  pronounced  meniscus  form 
and  bringing  them  closer  together, 
T.  Ross  made  the  objective  include 
a  still  wider  angle.  Owing  to  the 
proximity  of  the  diaphragm  to  the  lenses,  this  com- 
bination is  singularly  free  from  flare. 

Genesis  of  the  Doublet. — Before  dismissing  this  lens, 
we  present  drawings  of  both  the  original  and  the  last 
form  assumed  by  it.  Fig.  33  shows  the  objective  made 
in  1841  for  Henry  Collen. 
In  it  both  lenses  were 
plano-convex;  they  were 
separated  by  a  consider- 
able space,  had  a  rather 
small  diaphragm  in  the 
centre,  and  gave  a  round 
field,  so  much  so  that  when  taking  portraits  by  it  (for  it 
was  constructed  specially  for  portraiture)  it  was  neces- 
sary to  have  the  sensitive  negative  paper  pressed  in  shape 
between  two  glasses  bent  in  spherical  form.  The  latest 
form  of  that  doublet  is  shown  in  Fig.  34,  which  differs  from 
that  first  shown  by  having  its  elements  set  closer  together. 


FIG.  33. 


70  DA  VIDSOWS  COMBINA  TTON. 

The  doublet  was  in  all  cases  made  of  flint  and 
crown  glass,  and  hence  required  a  rather  small  dia- 
phragm ;  but  we  have  lately  seen 
some  lenses  formed  of  two  kinds 
of  flint  glass  and  figured  like 
that  shown  on  the  previous  page 
(see  Fig.  32),  which  work  with  an 
aperture  as  great  as  that  of  our 
most  modern  lenses. 

Davidson's  Combination. — In  1841, 
Thomas  Davidson,  a  well-known 
Edinburgh  optician,  constructed 
symmetrical  lenses,  concerning 
which  it  is  worthy  of  notice  that 
FIG'  34*  they  were  externally  similar  to  the 

most  approved  rapid  doublets  of  the  present  day.  Each 
lens  was  formed  of  a  plano-convex  crown,  cemented  at  its 
surface  to  a  plano-concave  flint.  We  had  a  lens  of  this  flat 
class  made  by  a  son  of  Davidson  more  than  a  quarter 
of  a  century  since,  and  so  well  did  it  work  that  it  is 
doubtful  if,  with  all  our  modern  appliances,  much  better 
pictures  can  be  taken  now  than  were  produced  by  this 
lens  invented  fifty  years  ago.  Why,  it  may  be  inquired, 
was  it  allowed  to  fall  into  a  state  of  desuetude  ?  We 
reply :  Davidson  introduced  it  as  a  portrait  lens,  for 
which  purpose  it  could  not  compete  with  the  Petzval 
portrait  combination  introduced  about  the  same  time 
by  Voigtlander.  The  processes  practised  in  those  days 
were  slow,  and  the  most  rapid  portrait  lens  was  that 
which  secured  preference. 


WIDE-ANGLE  APLANA  T. 


FIG.  35- 


Dallmeyer's  Wide-angle  Rectil  near. —  In  this   objective 
the  lenses  are   both  of  the  form  in   which   the  denser 

,  material  of  the  achromatic  lens  is 
placed  to  the  outside.  Although 
Dallmeyer  made  them  for  the 
most  part  as  shown  in  the  figure, 
that  is,  with  a  front  lens  of  larger 
diameter  and  longer  focus  than 
that  of  the  back  lens,  yet  are 
they  also  made  symmetrical, 
especially  those  of  short  focus. 
They  are  formed  of  flint  and 
crown  glass. 

Steinheil's  Wide-angle  Aplanat. 
—  Steinheil  having  recognised 
the  advantages  accruing  from  the  exclusive  employ- 
ment of  flint  glass  of  different  refractive  and  dis- 
persive ratios,  as  employed  in  his  rapid  aplanat, 
afterwards  constructed  one  on  the  same  general  system 
but  of  small  diameter  so  as  to  be  quite  portable.  The 
lenses  were  thicker  than  those  usually  made  of  similar 
diameter,  and  were  set  so  closely 
together  as  in  some  instances  to 
barely  allow  the  diaphragm  to 

-  be  inserted  between  them.  These 
lenses  include  a  very  wide  angle, 
and  are  quite  free  from  the  flare 
spot.      They  are   manufactured 
by  various  makers,  in  many  cases 

under   the  trade  designation  of 

FIG.  36. 


STEINHEWS   ANTIPLANET. 


the  Portable  Symmetrical,  which  was  first  given  them 
by  Ross  &  Co.,  although  other  makers  adopt  different 
names.  They  work  for  the  most  part  with  an  aperture 
equalling  a  sixteenth  of  their  focus.  A  distinguishing 
characteristic  of  the  Ross  portable  symmetrical  is  the 
identity  of  diameter  of  mounts  and  flanges  of  all  the 
usual  foci,  and  the  great  perfection  of  detail  given  by  it 
over  the  large  angle  included. 

Steinheil's  Antiplanet. — This  lens  partakes  of  the  nature 
of  the  orthoscopic  objective  to  this  extent,  that  the  front 
lens  is  a  positive  and  the  back  a  negative  combination, 
although  the  latter  is  so  to  only  a 
very  slight  extent.  It  will  be  seen 
from  the  cut  (Fig.  37)  that  the  back 
lens  possesses  an  unusual  degree  of 
thickness,  this  being  necessary  to  cor- 
rect the  aberrations  of  the  anterior 
combination.  Steinheil  makes  the 
antiplanet  in  two  forms,  one  having  a 
smaller  angular  aperture  than  the  other.  The  former, 
which  we  have  figured  here,  is  intended  for  groups,  &c. 
The  latter  is  a  portrait  lens,  which  works  at/-4,  and  it 
differs  from  the  group  lens  by  the  introduction  of  an  air 
space  between  the  elements  of  the  back  combination. 


FIG.   37. 


CHAPTER   XIII. 

PORTRAIT   LENSES. 

BY  a  portrait  lens,  or  combination  of  lenses,  is  meant 
jne  having  an  aperture  so  large  in  comparison  with  its 
focus  as  to  admit  a  volume  of  light  of  sufficient  intensity 
as  to  enable  portraits  to  be  taken  in  the  subdued  light 
of  a  studio  in  the  briefest  possible  period  of  time. 

Being  aplanatic,  a  portrait  lens  is  capable  of  defining 
sharply  without  any  diaphragm,  although,  as  we  shall 
eventually  show,  a  diaphragm  is  indispensable  for  se- 
curing its  full  advantages.  It  may  be  urged  that  any 
lens  by  which  a  portrait  is  capable  of  being  produced 
may  be  entitled  to  the  designation  of  a  '  portrait  lens,' 
but  in  technical  language  the  term  is  only  applicable 
to  those  of  a  certain  description,  between  which  and 
the  original  landscape  lens  there  are  now  so  many 
grades  as  to  render  somewhat  difficult  the  drawing  of 
a  hard  and  fast  line. 

History  of  the  Portrait  Lens.  —  The  portrait  combi- 
nation is  a  triumph  of  optical  skill,  and  in  its  original 
and  general  form  is  an  emanation  from  the  mathe- 
matician, Professor  Petzval,  of  Vienna.  The  history 
of  its  inception  may  be  told  in  a  few  words : —  In 


74  PETZVADS  PORTRAIT  LENS. 

1840  Professor  von  Ettingshausen,  having  returned 
from  a  visit  to  Paris,  where  the  daguerreotype  process 
was  engaging  the  attention  of  the  scientific  world, 
remarked  to  Petzval  that  Daguerre,  with  whom  he 
had  been  in  direct  intercourse,  made  use  of  a  lens 
having  a  small  diaphragm,  by  which  a  great  loss  of 
light  ensued,  and  inquired  if  he  (Petzval)  could  not 
devise  a  better  form  of  lens.  Acting  upon  this  hint 
Petzval  instituted  researches,  and  the  year  following 
(1841)  gave  to  Voigtlander — at  that  time  an  optician 
enjoying  a  high  reputation — the  formulae  for  two  ob- 
jectives, both  of  them  working  without  a  diaphragm. 
One  had  a  large  aperture  and  short  focus,  and  gave 
great  concentration  of  light  over  a  large  area ;  the 
other  had  a  longer  focus,  and  was  capable  of  covering 
a  large  field.  The  former  was  the  now  well-known  and 
universally-used  portrait  lens,  the  other  being  the  ortho- 
scopic,  which  was  allowed  to  lie  perdu  for  several  years 
afterwards.  A  becoming  distinction  not  having  at  that 
time  been  recognised  between  actinic  and  visual  achro- 
matism, the  lenses  of  early  tirr.cs  had  what  has  been 
succinctly  designated  a  '  chemical  focus  ' — a  fault  which 
is  now  eliminated  from  the  productions  of  every  lens 
manufacturer  of  eminence.  Thus  much  by  way  of 
remark  on  the  early  history  of  the  portrait  combination. 
What  is  Angular  Aperture? — The  leading  distinction 
between  the  portrait  and  other  lenses  is  implied  in 
the  term  '  angular  aperture.'  This  it  is  which  de- 
termines rapidity.  Angular  aperture  has  no  relation 
to  actual  size  or  diameter  of  lens,  except  so  far 


ANGULAR  APERTURE.  75 

as  such  relates  to  focal  length  ;  hence  a  lens  only 
one  inch  in  diameter  may  be  a  much  quicker-acting 
instrument  than  one  of  three  inches,  because  of  its 
aperture  being  larger  in  proportion  to  its  focus.  In 
making  choice  of  a  lens  for  rapidity  of  action  care  must, 
therefore,  be  taken  to  select  one  of  short  focus  in  pro- 
portion to  its  actual  diameter.  The  acting  angular 
aperture  of  a  lens  varies  with  every  different  stop  that 
is  used  ;  and  it  is  frequently  necessary  to  reduce  this 
aperture  considerably — not  for  the  sake  of  weakening 
the  light  and  thus  protracting  the  exposure,  but  in  ordei 
to  confer  a  greater  degree  of  penetrative  power,  foi 
'depth  of  focus'  varies  in  inverse  ratio  to  angular 
aperture.  When  a  comparison  of  lenses  is  made  in 
order  to  determine  which  is  the  better,  both  should  be 
as  near  as  possible  of  similar  diameter  and  focus ;  because 
two  lenses  may  be  of  the  same  diameter — say  three  inches 
— but  one  of  them  having  a  focus  of  six  inches  and  the 
other  of  twelve  inches,  the  difference  between  the  two 
as  regards  rapidity  will  be  this — that  the  one  of  twelve 
inches  will  necessitate  an  exposure  four  times  longer 
than  that  required  by  the  other  in  order  to  obtain 
equally  exposed  negatives.  Again :  two  lenses  may 
have  the  same  focus,  one  of  them  having  a  diameter 
of  three  inches,  while  that  of  the  other  is  only  one  inch 
and  a  half.  The  former  possesses  four  times  the  in- 
tensity of  the  latter,  and  will  work  in  a  fourth  of  its 
time.  A  just  comparison  cannot  be  made  between 
two  lenses  of  the  same  focus  but  dissimilar  dimensions 
unless  both  are  stopped  to  the  same  extent 


76  PORTRAIT  LENSES. 

The  portrait  objective  consists,  as  shown  in  the  ad- 
joining diagram  (Fig.  38),  of  two  achromatic  lenses  of 
dissimilar  form  mounted  at  some  distance  apart.  The 
anterior  lens  is  a  plano-convex,  or,  more  usually,  a 
meniscus  of  such  a  slight  external  concave  curvature 
as  to  seem  to  a  cursory  observer  to  be  plane.  Its 
component  parts  are  a  crown  glass  double  -  convex, 


FIG.  38. 

attached  by  transparent  cement  to  a  piano  -  convex 
flint  lens.  The  posterior  lens  is  a  double-convex  com- 
posed of  a  bi-convex  crown  and  a  concavo-convex 
of  flint  glass.  The  inner  curves  of  these  are  not  con- 
centric, as  in  the  anterior  lens.  They  are  usually 
mounted  so  as  not  to  touch  each  other,  and  when 
tested  as  a  whole  will  be  found  lacking  in  the  power  of 
bringing  rays  to  a  sharp,  or  even  moderately  sharp,  focus. 
Properties  of  Back  Combination. —  The  back  combi- 
nation of  a  portrait  lens  fulfils  a  twofold  function : 


&ACK  COMBINATIONS.  '17 

it  shortens  the  focus,  and  thus  aids  in  conferring  in- 
tensity of  illumination  ;  it  also  distributes  over  a 
flatter  field  the  image  formed  by  the  anterior  lens, 
which,  without  the  correcting  influence  of  the  back 
lens,  would  be  sharp  only  over  a  very  limited  area. 
This  is  the  principal  function  of  the  back  lens,  and 
it  performs  it  because  of  its  excess  of  negative  sphe- 
rical aberration  —  a  property  that  will  be  observed 
readily  if  the  posterior  combination  be  employed  as 
a  magnifier  in  the  examination  of  any  printed  matter, 
when  it  will  be  found  that  the  focus  of  the  centre 
is  shorter  to  a  considerable  extent  than  that  of  the 
margin.  Seeing  that  this  property  of  negative  aberra- 
tion is  modified  by  the  distance  apart  of  the  elementary 
components  of  the  posterior  lens,  it  is  frequently  possible 
to  convert  a  bad  lens  into  a  good  one  by  a  slight  ad- 
justment of  this  portion  of  the  objective.  Many  portrait 
combinations  have  the  back  lenses  placed  loosely  in  the 
cell,  with  a  flat  ring  of  brass  between  to  keep  them 
apart.  An  objective  of  this  class,  four  and  a  half  inches 
in  diameter,  intended  for  15  x  12  negatives,  which  per- 
formed very  badly  in  consequence  of  its  roundness  of 
field,  the  centre  of  the  picture  only  being  sharp,  had 
the  separating  ring  of  the  back  components  entirely 
removed,  and  with  marked  advantage.  This  posterior 
combination  was  now  found  to  have  its  negative  aber- 
ration greatly  increased,  for  the  separating  ring  was 
half  an  inch  in  width.  Now,  as  the  anterior  lens  of 
the  objective  was  of  much  shorter  focus  than  the  back 
.one,  it  was  considered  necessary,  in  consequence  of  the 


78  DALLMEYEKS  BACK  LENS. 

now  increased  negative  aberration  of  the  back,  to  bring 
the  front  and  back  lenses  much  closer  together.  Ac- 
cordingly, after  a  few  trials  the  tube  was  shortened  to 
the  extent  of  an  inch  and  three-quarters,  with  the 
gratifying  result  of  the  objective  working  in  an  exceed- 
ingly satisfactory  manner,  and  taking  a  sharp  portrait 
on  a  15  x  12  plate — the  full  size  it  was  intended  to 
cover.  This  incident  is  mentioned  because  a  bad  lens 
was  converted  into  a  good  one  without  a  necessity 
being  experienced  for  regrinding  any  of  the  surfaces. 
Dallmeyer's  Back  Lens. — A  form  of  back  lens,  dif- 
fering from  that  of  Petzval,  was  introduced  several 
years  ago  (1866)  by  J.  H.  Dallmeyer,  in  which  both  the 
forms  and  the  relative  positions  of  the  components 


FIG.  39. 

are  reversed.  Its  nature  will  be  ascertained  from  the 
diagram  (Fig.  39),  in  which  the  back  lens  is  seen 
to  consist  of  a  shallow  meniscus  formed  of  a  con-* 


WATERHOUSE  DIAPHRAGMS.  ^9 

cavo-convex  of  flint  (the  convex  side  being  nearest 
the  ground  glass)  and  a  meniscus  of  crown.  The  two 
lens  are  so  constructed  that  when  placed  as  closely 
in  contact  as  possible  the  objective  will  give  sharp 
definition,  but  when  separated  in  even  a  very  slight 
degree  spherical  aberration  is  introduced  to  any  desired 
extent,  thus  lowering  the  definition.  This  form  of  back 
combination  is  now  adopted  by  some  of  the  leading 
Continental  opticians,  who  burnish  the  two  lenses  in 
one  cell,  thus  discarding  the  advantage  conferred  by 
separation  of  the  constituents. 

Waterhouse  Diaphragms, — All  portrait  objectives  of 
any  pretensions  to  the  highest  quality  are  now  fitted 
with  diaphragms.  At  first  these  were  inserted  in  the 
hood  of  the  lens,  and  kept  in  their  place  by  a  ring 
the  width  of  the  hood.  It  then  occurred  to  Mr.  Lake 
Price  to  slit  the  tube  so  as  to  drop  in  one  of  a  series 
of  loose  diaphragms  between  the  lenses  ;  but  the  in- 
vention is  now  associated  with  the  name  of  Dr.  Water- 
house,  who  further  simplified  the  system. 

Discoloured  Glass. — We  have  spoken  of  angular 
aperture  as  the  great  requisite  towards  rapidity ;  but 
there  is  another  which,  while  less  essential,  is  of  great 
importance.  We  refer  to  quality  of  glass.  Both  crown 
and  flint  optical  glass  are  sometimes  apt  to  be  a  little 
'off'  the  colour  even  when  made,  and  it  is  a  well- 
known  fact  that  discoloration  occurs  in  some  lenses  by 
merely  exposing  them  1o  a  strong  light.  This  will 
be  more  specially  alluded  to  in  a  subsequent  chapter. 


CHAPTER  XIV. 

RAPID  LANDSCAPE,  GROUP,  AND  COPYING  LENSES. 

Nature  of  a  Rapid  Lens, — What  constitutes  a  rapid  lens 
is  not  very  easy  to  define.  That  a  portrait  combination, 
having  a  large  angular  aperture,  is  really  the  most  rapid 
worker  of  all  no  one  can  for  a  moment  entertain  any 
doubt ;  and  yet  it  is  not  '  rapid  '  in  the  sense  in  which 
we  have  now  to  speak  of  the  instrument,  but  must  be 
suffered  to  remain  outstanding,  and  yield  the  phraseo- 
logical distinction  to  others  much  slower. 

The  term,  first  introduced  by  Mr.  Dallmeyer  to 
distinguish  one  of  his  rectilinears,  may  be  considered  as 
now  applying  to  combinations  constructed  for  the 
purpose  of  including  a  wider  angle  than  the  portrait 
lens  on  the  one  hand,  and  a  smaller  angle,  on  the 
other,  than  can  so  easily  be  obtained  by  the  wide- 
angle,  non-distorting  lenses  which  were  described  in  the 
preceding  chapter.  Any  combination  which  will  in- 
clude a  moderate  angle  of  view,  such  as  two-thirds  of 
its  focus,  with  an  aperture  from  f-6  to/-io,  and  be  free 
from  distortion,  is  entitled  to  be  considered  a  '  rapid '  or 
aplanatic  lens. 

The  first  of  this  class  of  which  we  possess  any  record 


RAPID  RECTILINEARS.  81 

was  issued  in  July,  1866,  by  the  late  Dr.  Steinheil,  at  the 
suggestion  of  the  late  Dr.  Monckhoven,  who  supplied 
the  required  data  which  should  be  kept  in  view  in  the 
construction  of  such  a  lens  as  was  at  that  time  con- 
sidered a  desideratum.  The  instrument  which  resulted 
from  a  conference  between  the  two  savants  possessed 
an  aperture  equalling  one-seventh  of  the  focus.  It  was 
formed  of  two  different  kinds  of  flint  glass.  But  in  a 
patent  obtained  by  Mr.  J.  H.  Dallmeyer  about  the  time 
of  the  issuing  of  the  Steinheil  aplanatic  lens — as  the 
new  claimant  for  public  favour  was  designated — the 
principle  upon  which  this  lens  is  constructed  was  em- 
braced ;  for  in  the  specification  of  the  patent  which  has 
primary  reference  to  the  wide-angle  rectilinear  described 
and  figured  in  our  last  chapter,  together  with  the  back 
combination  of  his  portrait  objective,  and  which  patent 
was  obtained  in  the  course  of  the  year  above  mentioned, 
he  says  : — '  A  lens  may  be  constructed  according  to  my 
invention  of  flint  glass  only,  necessarily  of  two  dif- 
ferent kinds  as  regards  density  for  the  production  of 
achromaticity,  instead  of,  as  is  usual,  crown  and  flint 
glass.' 

There  is  ample  evidence  that  these  two  ivorkers  were 
employed  in  independent  investigations,  although  in  the 
matter  of  publication  Steinheil  had  the  priority. 

Modifications.  —  Although  the  general  principle  of 
construction  is  similar  in  all  of  the  *  rapid '  type  of 
lenses,  with  one  exception,  yet  several  modifications 
as  regards  curvature  and  densities  of  glass  have  been 
made  by  the  respective  manufacturers  of  this  rapid 


82 


ADVANTAGES  OF  DENSE  GLASS. 


doublet.  The  accompanying  diagram  (Fig.  40)  is 
sufficiently  accurate  to  describe  nearly  all  '  rapid '  lenses 
(with  the  one  exception  alluded  to)  by  whomsoever 
they  are  constructed.  Each  achro- 
matic lens  in  the  combination  is 
a  meniscus  formed  of  dense  glass, 
the  denser  element  forming  the  side 
that  is  convex.  The  elements  in 
each  are  a  concavo-convex  and  a 
meniscus  cemented  together,  and 
two  of  these  form  the  objective,  the 
apertures  of  which,  according  to 
FIG-  40.  the  maker,  may  be  considered  as 

varying  from  /-4  to  /- 10.  The  former  of  these,  however, 
implies  that  glass  has  been  made  use  of  having  a  degree 
of  density  scarcely  safe  to  be  employed  for  photographic 
lenses  on  account  of  its  tendency  to  become  discoloured. 
Advantage  of  Dense  Glass.  —  Why,  it  may  be  asked, 
employ  glass  of  such  great  density  ?  Or  what  advantage 
does  heavy,  dense  glass  possess  over  the  lighter  sort 
known  to  be  unalterable  by  either  light  or  time? 
We  reply :  the  denser  the  material  of  which  a  lens  is 
constructed  the  greater  is  its  refractive  power,  and, 
consequently,  the  flatter  is  the  curvature  required  to 
produce  a  lens  of  any  definite  focus.  We  here  repeat 
what  we  have  already  stated,  that  if  three  single  lenses 
are  required  of  similar  short  foci,  all  being  the  same 
diameter,  and  the  first  be  composed  of  diamond  (if 
that  were  practicable),  the  second  of  dense  flint  glass, 
and  the  third  of  light  crown  glass,  then,  while  the  first 


SYMMETRY.  83 

would  be  comparatively  flat,  the  last  would  be  very  thick, 
owing  to  its  short  radius  of  curvature,  while  the  second 
would  be  between  the  two.  Now,  seeing  that  the  radius 
of  curvature  of  a  dense  glass  is  so  much  greater,  for  its 
diameter  and  focus,  than  one  of  light  material,  the 
spherical  aberration  is  diminished  in  a  corresponding 
degree.  It  is  impossible  to  produce  with  ordinary  flint 
and  crown  glass  a  combination  of  the  form  shown  in  the 
foregoing  diagram  which  shall  work  with  an  aperture  as 
great  as  those  formed  of  dense  glass.  Hence  the  advan- 
tage of  the  latter  kind  of  glass. 

Symmetry. — Symmetry  in  a  rapid  doublet  (by  which 
name  we  shall  designate  this  class  of  lens,  by  whom- 
soever manufactured)  is  not  at  all  a  requisite  condition 
towards  obtaining  either  a  large  angular  aperture, 
covering  power,  or  rectilinearity  of  projection.  Some 
years  ago  a  statement  was  made  by  the  author  to  the 
effect  that  for  all  purposes,  except  that  of  copying  an 
object  the  size  of  the  original,  the  lenses  of  a  rapid 
doublet,  examined  from  the  non-distorting  point  of  view, 
should  not  be  symmetrical.  This  drew  forth,  first,  the 
strong  animadversions  of  the  deceased  Thomas  Sutton, 
whose  mathematical  ability  no  one  doubts  ;  and,  secondly, 
an  adverse  private  expression  of  opinion  from  the  then 
mathematical  adviser  of  a  large  optical  firm  who  now 
in  practice  ignore  strict  symmetry.  Such  is  the  irony 
of  fate  !  A  vast  number  of  the  rapid  doublets  now 
being  manufactured  have  their  front  lenses  of  longer 
focus  than  their  backs.  This  dissimilarity  is  sometimes 
carried  so  far  as  to  cause  a  sensible  difference  in  focus 


84  MORRISONS  RAPID  DOUBLET. 

of  the  combination  when  the  full  aperture  and  a  small 
stop  are  respectively  employed.  The  reason  underlying 
this  dissimilarity  of  elements  in  an  objective  have  relation 
to  the  law  of  conjugate  foci.  But  photographic  optics  is 
so  much  a  series  of  compromises  that  it  is  unwise  to 
dogmatise  upon  what  should  be  the  way  to  carry  into 
effect  a  certain  idea,  as  it  is  impossible  to  indicate  any 
one  mode  as  being  the  best.  The  form  of  rapid  doublet 
shown  in  Fig.  40  (ante)  is  that  which  has  been  adopted  by 
all  European  manufacturers,  and  it  is  a  necessity  of  their 
construction  that  glass  of  greater  than  ordinary  density 
be  employed  in  their  formation.  It  may  be  an  abnor- 
mally dense  crown  glass  united  with  flint  glass  of  a 
corresponding  ratio  of  density  to  secure  the  requisite 
actinic  correction  ;  or  it  may  be  a  light  flint  glass  com- 
bined with  heavy  flint,  the  result  being  the  same. 

Morrison's  Rapid  Doublet.  -  -  The  rapid  doublet  of 
Richard  Morrison,  an  American  manufacturing  photo- 
graphic optician,  formerly  spoken  of  and  lately  deceased, 
appears  to  have  been  projected  on  lines  totally  different 
from  those  of  European  opticians  ;  for,  not  only  is  it 
formed  of  the  ordinary  optical  flint  and  crown,  but  the 
very  principles  involved  in  its  manner  of  correction  differ 
from  them.  In  Fig.  41  we  present  a  diagram  of  this 
.\merican  rapid  doublet,  the  curves  of  which  are  none 
of  them  deep  in  any  part,  differing  in  this  respect  from 
the  internal  or  contact  surfaces  of  the  European  class, 
the  radius  of  which  is  always  very  short.  From  what 
\ve  have  seen  of  this  American  objective  when  tried 
in  comparison  with  those  of  the  European  form  there 


•  RAPW  DOUBLET.  85 

does  not  appear  to  be  much  difference  between  them. 
There  are  numerous  particular  instances  in  both  classes 
in  which  one  has  proved  much  superior  to  the  other  ; 


FIG.  41. 

but  in  the  best  specimens  of  each  the  difference  between 
the  photographic  results  is  not  readily  apparent.  A 
priori,  the  European  form  should  possess  such  an  ad- 
vantage over  the  American  as  is  to  be  obtained  from 
the  reflecting  surfaces  being  only  half  the  number ;  for 
the  interior  surfaces  of  the  Morrison  lenses  being  dis- 
similar as  regards  curvature,  it  is,  of  course,  impossible 
that  they  can  be  cemented.  This  in  practice,  however, 
is  not  a  matter  of  the  importance  that  might  at  first  be 
imagined  from  the  '  loss  of  light '  point  of  view,  because 
a  very  slightly  increased  diameter  of  lens  will  amply 
compensate  this. 

Where  the  real  ^>oint  of  danger  is  apt  to  lie,  if  care 
be  not  taken  in  properly  adjusting  their  various  parts 
is  in  the  increased  number  of  images  formed  along  the 
posterior  axis  by  these  various  reflecting  surfaces.  The 
Morrison  rapid  doublet,  if  gifted  with  speech,  might 
r  r 


86  MORRISONS  RAPID  DOUBLET. 

hurl  a  tu  quoque  against  its  European  rivals  ;  for  it  is 
the  case  that  by  many  of  the  rapid  doublets  a  cjntral 
flare  spot  will  be  produced  if  the  conditions  are  such 
as  to  favour  its  production. 

What  we  have  said  regarding  this  objective  com- 
paring favourable  with  the  European  rectilinears,  must 
be  held  as  applying  to  narrow  angles  of  view  only ;  for, 
as  might  be  deduced  from  a  perception  of  its  shallow 
curves,  the  Morrison  doublet  cannot,  from  the  very 
nature  of  its  construction,  transmit  an  oblique  pencil 
in  the  perfection  capable  of  being  attained  by  the 
cemented  combinations  of  European  form  just  de- 
scribed ;  hence  for  including  other  than  a  narrow 
angle  of  view  it  must  yield  the  palm  to  them. 


CHAPTER  XV. 

UNIVERSAL   LANDSCAPE  LENSES. 

What  Constitutes  a  Universal  Lens. — By  '  universal,'  in 
the  above  heading,  is  here  meant  adaptability  or  adjust- 
ability of  focus.  The  photographer  has  his  camera 
pitched  at  the  one  point  from  which  alone  the  composi- 
tion of  the  subject  is  perfect,  but  when  focussed  upon 
the  ground  glass  it  is  found  that  either  too  much  or  too 
little  of  the  scene  has  been  got  in.  Then  why  not  carry 
a  battery  of  lenses,  so  that  when  one  fails  in  delineating 
upon  the  ground  glass  just  so  much  as  is  wanted  and  no 
more,  it  may  be  deposed  in  favour  of  another  which  will 
better  fulfil  the  requirements  of  artistic  composition  ? 
While  such  an  expedient  is  to  the  individual  possessing 
ample  means  the  most  satisfactory  that  could  be  adopted, 
it  is  open  to  the  serious  objection  of  great  expense  and 
much  bulk — especially  the  former.  Having  one  mount 
it  is,  of  course,  easy  to  adapt  to  it  a  variety  of  lenses 
set  in  cells,  each  lens  either  set  far  back  or  made  to 
project  in  its  cell  according  to  its  focus  ;  for  it  is  scarcely 
necessary  to  remark  that  the  longer  the  focus  of  the  lens 
the  greater  must  be  its  distance,  cceteris  paribus,  from 
the  stop. 

Convenience  of  the  Universal  System. — This  system  is 
much  to  be  commended,  as  it  enables  the  photographer 


88  CA SKE  T  LENSES. 

to  reduce  his  impedimenta  to  a  considerable  extent 
without  having  to  sacrifice  efficiency  or  convenience  in 
any  degree.  During  a  series  of  discussions  on  land- 
scape lenses  which  took  place  at  the  Photographic  Club, 
the  author,  speaking  on  this  subject,  showed  a  mount  of 
convenient  dimensions  to  which  he  had,  by  suitable 
adapters,  fitted  lenses  by  Grubb,  Ross,  Dallmeyer, 
Darlot,  and  others.  These  packed  into  a  pocket-case  by 
themselves  ;  and  by  making  a  selection  he  could  have 
every  focus,  either  singly  or  in  combination,  for  which 
his  camera  was  adapted.  These  were  not  mere  make- 
shifts, but  each  was  adjusted  according  to  strict  rule. 
Many  years  since  M.  Darlot,  a  Continental  manufacturer, 
devised  and  executed  a  cabinet  of  lenses  for  a  similar 
purpose.  Casket  lenses  are  now  being  made  by  several 
manufacturers. 

Universal  Lens  on  New  System.— Perhaps  the  most 
useful  lens  of  all,  should  it  ever  reach  the  stage  of  being 
manufactured,  will  be  that  which  was  referred  to  by  the 
author  as  having  been  devised  by  him,  but  as  yet  in  a 
too  unfinished  state  for  detailed  publication,  namely,  one 
in  which,  by  the  rotation  of  a  collar  or  the  movement  of 
a  button  in  a  slot  in  the  mount,  the  focus  of  the  lens- 
complete  in  itself — is  susceptible  of  being  altered  to  a 
considerable  extent.  That  such  really  can  be  done 
there  is  no  room  for  doubt,  as  we  have  made  use  of  such 
a  combination,  constructed  somewhat  roughly,  but  suffi- 
ciently well  to  show  the  action.  The  alteration  of  the 
focus  is  caused  by  the  movement  to  and  fro  of  certain 
lenses,  more  especially  of  a  concave  achromatic,  so  con- 


FOCUS  ADJUSTER.  89 

structed  as  not  to  interfere  with  chromatic  correction  no 
matter  how  effected.  A  principle  analogous  to  this  has 
for  some  time  been  applied  to  a  low-power  microscopic 
objective  by  Carl  Zeiss,  Wray,  and  others. 

A  Focus  Adjuster.  —  A  convenient  form  of  focus 
adjuster,  which  we  devised  and  had  constructed  several 
years  ago,  consists  in  a  sliding  piece  of  brass,  made 


FIG.  42. 

hollow  in  order  to  secure  lightness,  of  the  form  shown  in 
Fig.  42.  It  contains  four  apertures,  into  each  of  which 
is  fitted  a  thin  achromatised  lens  of  negative  power. 
This  piece  slides  through  the  lens  mount,  by  means  of 
an  aperture,  shown  in  Fig.  43. 
There  are  a  series  of  notches  on 
"^Wfcr  the  slide  so  as  to  ensure  the  lens 

|J— ]      V$j{ 

PJJ  -HI          connected  there v/ith  being  kept 

quite  central.     The  combination 
to  which  this  system  is  accached 
is  a  doublet    composed   of  two 
\\  slightly  meniscus  lenses  which, 

r  ~"  O^i    w^en  use<^  al°ne>  do  not  give  a 

flat  field.     By  inserting  the  slide 
the  influence  of  either  of  the  four 
concave  lenses  contained  in  it  is  to  flatten  the  field  and 


96  FOCUS  ADJUSTER. 

lengthen  the  focus — the  marginal  pencils  being  well 
corrected  with  a  moderately  large  aperture.  With 
i\To.  I  lens  the  equivalent  focus  is  seven  inches,  the 
other  concaves  increasing  the  focus  respectively  in  the 
following  proportions  : — 

No.  i pinches. 

„  2 9      „ 

»  3 12      „ 

»  4 !5     » 

When  not  in  use  this  slide  packs  away  in  a  neat  little 
pocket-case,  six  inches  long  by  one  and  a  half  inches 
wide,  and  half  an  inch  deep.  This  forms  a  compact  and 
useful  appendage  to  a  lens.  If  one  of  the  lenses  of  the 
combination  be  removed  an  entire  change  of  focus  is 
produced  ;  but  in  this  case  it  is  lengthened  so  much  as 
to  be  useless  when  employed  with  a  small  camera.  A 
series  of  three  auxiliary  lenses  mounted  in  similar 
fashion  was  prepared  and  long  used  by  us  in  connexion 
with  the  Petzval  orthoscopic  system,  the  performance 
being  so  good  as  to  have  elicited  from  a  clever  manu- 
facturing optician  an  expression  of  surprise  at  what  he 
termed  the  great  adaptability  and  elasticity  of  this 
system. 

Every  one  knows  that  there  is  a  horn  or  shell  pocket 
magnifier  which  can  be  obtained  for  a  few  shillings,  and 
which  consists  of  three  lenses  of  different  powers  set  in 
horn  and  hinged  on  a  common  pivot,  so  as  to  rotate  in 
or  out  as  required.  These  lenses  being  of  different  foci 
form  a  tiny  battery  of  seven  degrees  of  magnifying 


ELEMENTS  OF  COMBINATIONS.  91 

power,  according  as  they  are  employed  singly  or  in  com- 
bination with  one  another  ;  and  something  analogous  in 
principle  to  this  in  photographic  lenses  is  what  we 
contend  for  as  a  tool  that  would  prove  highly  useful  to 
landscape  photographers.  There  is  much  optical  talent 
lying  dormant  among  photographers.  We  trust  that 
what  has  been  here  said  will  prove  the  means  by  which 
some  of  this  inert  power  may  be  aroused. 

The  Elements  of  Combinations  may  be  used  as  Single 
Landscape  Lenses. — In  connexion  with  this  subject  we 
may  remind  photographers  who  employ  combinations 
of  lenses,  such  as  those  of  the  rectilinear  or  symmetrical 
class,  that  each  lens  may  be  used  singly  as  well  as  in 
combination.  The  focus  will  then  be  about  twice  that  of 
the  complete  objective.  But  this  is  not  always  the  case,  as 
many  lenses  of  this  class  are  dissimilar,  the  front  being 
of  longer  focus  than  the  back.  This  is  all  the  better  as 
regards  diversity,  as  it  affords  three  changes.  But  when 
employing  only  one  of  the  elements  of  this  objective  as 
a  single  landscape  lens  the  best  effect  is  not  obtained  if 
the  lens  be  screwed  into  the  tube  in  the  usual  way.  It 
is  then  rather  too  close  to  the  stop.  But  by  having  an 
adapting  ring  into  which  it  can  be  screwed,  so  as  to 
allow  of  a  greater  distance  between  it  and  the  diaphragm, 
its  full  value  will  then  be  ascertained.  The  central 
definition  will  be  good  under  all  circumstances  ;  but 
when  the  stop  is  close  to  the  lens  the  marginal  definition 
is  bad,  but  will  improve  in  proportion  as  the  space 
between  the  stop  and  lens  is  increased,  until  it  reaches 
the  maximum  extent  of  improvement.  One  such 


92  ELEMENTS  OF  COMBINATIONS. 

adapting  ring  (which  we  have  had  made  to  adapt  to  the 
single  lens  of  a  combination)  of  one  inch  and  five-eighths 
in  diameter,  possesses  a  width  of  three-quarters  of  an 
inch.  When  the  objective  employed  in  its  completed 
state  is  a  double  combination  everything  is  right ;  but 
when  the  front  lens  is  removed  then  the  stop  is  found  to 
be  three-quarters  of  an  inch  too  near  to  the  remaining 
lens  to  produce  the  flattest  field  when  using  it  alone. 

Incidentally  and  apropos  of  what  has  just  been  said  in 
relation  to  increasing  the  flatness  of  field  by  placing  the 
stop  at  the  proper  distance  in  front  of  the  lens,  we  may 
here  remark  that  sometimes,  even  when  making  use  of  a 
single  achromatic  lens,  a  flare  spot  is  found  on  the  centre 
of  the  plate.  This  has  been  denied  by  some;  but  the 
fact  remains  that,  under  certain  circumstances,  some 
single  achromatic  lenses  do  offend  in  the  manner  indi- 
cated. This  subject  is  more  fully  treated  in  the  chapter 
on  flare  and  the  flare  spot,  in  which  the  remedy  is 
described. 

If  the  combination  which  is  to  be  separated  for  the 
purpose  of  employing  only  one  of  the  lenses  be  a  wide- 
angle  one,  then  the  back  lens  may  be  removed  and  the 
front  one  left  in  situ,  convex  surface  to  the  view.  This 
is  an  entire  reversal  of  the  circumstances  under  which  a 
landscape  lens  is  usually  employed  ;  but  in  the  case  of 
the  lens  just  indicated  it  will  prove  best,  especially  if  the 
angle  to  be  included  is  not  great. 


CHAPTER  XVI. 

FLARE  AND  THE  FLARE  SPOT. 

FLARE  may  be  described  in  general  terms  as 
an  abnormal  transmission  of  light  through  the  lens 
whereby  the  brilliance  of  the  image  is  impaired.  It 
is  sometimes  caused  by  reflection  from  the  mount  of 
the  lens,  and  more  usually  by  reflections  from  the  lens 
itself. 

Flare  from  Imperfect  Mounting. — In  some  objectives 
the  lens  is  retained  in  its  cell  by  a  counter  screw  formed 
of  a  short  piece  of  tube  having  a  thread  on  its  outside, 
its  inside  being  blackened,  occasionally  by  staining  the 
metal,  and  not  unfrequently  by  means  of  a  coating  of 
dead  black  varnish.  The  former  of  these  is  altogether 
bad.  To  realise  this  it  merely  suffices  to  point  the 
camera  towards  a  brightly  lighted  scene,  and,  having 
laid  aside  the  ground  glass  and  thrown  a  large  focussing 
cloth  over  the  head,  direct  the  eyes  towards  the  mount 
of  the  lens  to  observe  what  an  amount  of  light  is  re- 
flected from  the  various  parts  of  the  setting.  Then  let 
it  be  remembered  that  all  such  reflected  light  thus 
observed  will  fall  upon  the  sensitive  plate  and  degrade 
the  brilliance  of  the  image. 


9*  FLARE  FROM  THE  MOUNT. 

When  the  counter  screws  are  finished  with  dead 
black  varnish,  there  is  but  little  light  reflected  at  first ; 
but  after  a  while,  when  the  surface  of  the  lens  has  been 
frequently  wiped  with  a  soft  cloth  to  free  it  from  dust, 
the  action  of  the  cloth  upon  the  black  varnish  of  the  cell 
ultimately  converts  the  dead  surface  into  a  shining  one 
which  is  a  powerful  reflector  of  light. 

In  some  of  the  lowest  priced  objectives  the  lens  is 
dropped  into  a  recess  at  the  end  of  the  mount  and  is 
retained  in  its  place  by  a  ring  screwed  in.  This  is  a 
more  fertile  source  of  flare  resulting  from  mounting 
than  any  other.  We  have  known  an  offensive  flare 
produced  in  a  landscape  lens  by  a  high-class  maker  by 
the  hollowing  of  the  cell  around  and  outside  of  the 
lens,  which  after  its  dead  black  varnish  got  brightened 
by  cleaning  the  convex  surface  of  the  glass  with  a  wash- 
leather,  reflected  as  would  a  parabolic  reflector  the  light 
radiated  from  the  surface  of  the  sensitive  plate. 

The  Eemedy  for  Flare  from  Mounting. — By  coating 
the  brass  work  with  dead  black  varnish,  a  receipt  for 
which  will  be  found  in  another  chapter,  flare  of  the 
nature  described  will  be  greatly  diminished  if  not  en- 
tirely cured.  The  edges  of  all  lenses  should  also  be 
blackened  previous  to  their  being  set  in  their  cells  ;  this 
is  done  by  applying  the  black  varnish  by  means  of  a 
camel's  hair  pencil. 

The  Optical  Flare  Spot. — No  lens,  not  even  one  of  the 
simplest  class,  has  ever  been  made  that  does  not  give 
two  images  of  any  luminous  body  in  front.  One  of 
these  is,  of  course,  the  primary  image  formed  at  the 


CAUSE  OF  THE  FLARE  SPOT.  $5 

principal  focus  ;  but  there  is  another  which  is  to  be  found 
in  the  axis,  and  usually  very  close  to  the  posterior 
surface  of  the  lens. 

Take  any  lens,  a  common  reading-glass  for  instance, 
and  interpose  it  between  the  flame  of  a  lamp  or  gas. 
Now  look  at  the  lens  with  both  eyes,  and  a  small, 
bright,  and  inverted  image  of  the  flame  will  be  seen  at  a 
distance  of  an  inch,  more  or  less,  from  the  lens.  It  is 
very  easy  to  locate  its  precise  position  and  to  receive  the 
image  upon  a  small  bit  of  tissue  paper  or  ground  glass  ; 
while,  if  desired,  the  primary  image  of  the  flame  may  be 
simultaneously  received  at  the  principal  focus  farther 
back.  Now,  an  achromatic  lens  gives  a  small  secondary 
image  just  the  same  as  does  the  reading-glass,  and 
arises  from  the  same  cause.  What  we  wish  the  reader 
to  bear  in  mind  at  present  is  the  fact  that  the  relation 
of  the  small  image  to  the  gas  flame  is  that  of  conjugate 
foci,  demonstrated  by  causing  the  lens  to  approach  close 
to  or  recede  from  the  flame,  when  the  image  changes  its 
position  accordingly. 

Cause  of  the  Secondary  Image. — Most  of  the  rays  are 
transmitted  through  the  lens  to  the  principal  focus,  but 
a  few  are  arrested  by  the  back  surface,  and  are  reflected 
to  the  anterior  surface  only  to  be  re-reflected  back  again 
and  transmitted.  The  result  of  the  refraction  and  re- 
flections they  undergo  is  to  bring  them  to  a  focus  quite 
close  to  the  lens.  Of  those  rays  which  do  not  undergo 
the  reflection  from  the  front  surface,  but  which  come 
to  a  focus  on  the  opposite  side,  we  shall  presently 
speak. 


g6  FLARE  IN  COMPOUND  LENSES. 

The  Flare  Spot  in  Landscape  Lenses. — When  adiaphragm 
is  placed  before  a  lens,  the  aperture  therein  has  the 
same  relationship  to  it  as  had  the  gas  flame  in  the 
former  case  ;  that  is  to  say,  the  small  bright  area  of  the 
stop  will  be  reproduced  as  a  circular  spot  of  diminished 
brightness  behind  the  lens.  As  this  has  a  conjugate 
relation  to  the  lens,  it  is  possible  by  bringing  the 
diaphragm  moderately  close  to  the  objective  to  form 
an  image  of  the  aperture  at  the  primary  focus,  or  upon 
the  sensitive  plate.  But  as  a  very  slight  alteration  in 
the  position  of  the  anterior  conjugate  (the  diaphragm) 
makes  a  great  difference  in  the  posterior  one,  it  merely 
suffices  to  make  such  slight  alteration  in  order  to  effect 
a  cure.  In  some  cases  such  a  trivial  alteration  as  an 
eighth  to  a  quarter  of  an  inch  suffices  to  convert  a  bad 
lens  into  a  good  one,  as  it  may  bring  the  ghostly  imigc 
forward  from  the  plate  to  a  position  near  to  the  lens 
whence  it  is  distributed  over  the  the  entire  surface  in  a 
state  so  attenuated  as  to  be  harmless. 

Flare  in  Compound  Leases. — In  proportion  to  the 
number  of  reflecting  surfaces  in  a  combination  so  does 
the  number  of  false  images  increase.  Let  a  Petzval 
portrait  combination  be  taken  into  a  darkened  room 
and  directed  to  a  lamp,  and  it  will  be  found  that  along 
its  axis  no  fewer  than  fourteen  images  of  the  flame  wiil 
be  seen,  four  of  them  erect,  and  ten  inverted.  Of  all 
combinations  this  one  seems  the  worst  to  work  with  a 
diaphragm  and  escape  the  presence  of  a  more  or  lc:>s 
bright  flare  spot.  To  avoid  this  evil  it  is  much  the 
better  way,  when  using  it  out  of  doors  with  a  bright  sky 


FLARE  IN  RECTILINEARS  97 

in  front,  not  to  employ  any  stop  at  all,  but  to  use  it  with 
full  aperture. 

Flare  in  Rapid  Rectilinears. — In  cemented  doublets  of 
the  '  rapid  '  type,'  now  in  such  general  use,  it  will  be 
found,  when  directing  it  to  a  lamp  or  gas  flame,  as  in 
the  previous  experiment,  that  the  number  of  reflected 
images  is  reduced  to  five  or  occasionally  to  six.  Of 
these,  one  depends  to  a  greater  extent  than  the  others 
upon  the  degree  of  the  separation  of  the  front  and  back 
components,  and  there  is  a  special  distance  at  which 
they  may  be  separated  where  the  concave  surface  of 
the  front  lens  will  be  seen  to  be  one  blaze  of  light.  To 
show  that  this  arises  from  reflections  from  the  back  lens, 
it  is  only  necessary  to  increase  or  decrease  the  amount  of 
separation  ever  so  slightly  to  cause  it  to  disappear.  The 
flare  spot  in  this  class  of  lens  is  most  pronounced  when 
the  distance  at  which  the  lenses  are  separated  is  such 
as  to  give  this  reflection,  the  relation  between  the 
diaphragm  and  the  back  lens  also  being  such  as  to  have 
the  image  of  the  former  thrown  on  the  sensitive  plate. 

To  Ascertain  whether  a  Lens  gives  FUre. — A  good 
way  by  which  to  discover  the  presence  of  this  flare 
propensity  in  any  lens  is  to  screw  it  into  a  camera 
and  focus  a  view  of  an  ordinary  gas  flame  on  the 
screen,  the  room  being  otherwise  darkened.  This  image 
will  be  sharp,  bright,  and  inverted.  Now  move  the 
camera  slightly  so  as  to  cause  the  inverted  image 
to  be  a  little  to  one  side  of  the  centre  of  the  focussing- 
screen,  and  in  nine  cases  out  of  ten  there  will  be  seen  a 
ghostly  image  at  the  opposite  side  of  the  centre.  This 

H 


98  CURING  FLARE. 

secondary  image  is  non-inverted,  and  upon  rotating 
the  camera  it  moves  in  the  opposite  direction  to  the 
primary  image.  The  nature  of  this  secondary  image 
and  the  cause  of  its  formation  may  be  examined  in 
the  following  way :  move  the  camera  so  that  the 
ghostly  image  shall  be  near  the  margin,  and  then, 
placing  the  eye  in  the  line  of  that  image  and  the  lens, 
withdraw  the  ground  glass,  when  the  posterior  surface 
of  the  lens  will  be  found  to  be  quite  luminous. 

That  the  false  image  is,  in  this  case,  caused  by  a 
reflection  from  the  back  surface  of  the  anterior  lens 
is  demonstrable  by  unscrewing  the  cell  containing  it 
until  it  is  almost  ready  to  drop  out  of  its  tube,  and 
then,  keeping  an  eye  upon  both  the  primary  and  the 
secondary  images  on  the  ground  glass,  move  or  slightly 
wriggle  the  front  cell,  which,  by  its  looseness  in  the 
mount  may  now  be  easily  done,  when  it  will  be  seen 
that,  while  the  primary  or  legitimate  image  of  the  flame 
remains  motionless,  the  flare  image,  caused  by  the  re- 
flection from  the  surface  of  the  front  lens,  dances  about 
all  over  the  plate.  But  observe,  further,  that  there  is  a 
certain  distance  between  the  front  and  back  lenses  at 
which  this  secondary  image  is  sharp  and  bright ;  and 
in  proportion  as  either  the  front  or  the  back  lens  cell 
is  screwed  out  or  in,  so  does  the  image  become  more 
attenuated  and  expanded  till  at  last  it  ceases  to  be  seen 
altogether,  while  all  this  time  the  real  image  is  not;  seen 
to  suffer  in  any  way. 

The  Cure. — This  tendency  of  the  ghostly  image  to 
pass  out  of  focus  with  such  extreme  rapidity  upon 


FLARE.  99 

separating  the  lenses  by  a  few  turns  of  the  screw,  or 
even  by  making  them  come  nearer  to  each  other,  pro- 
vides the  means  by  which  this  annoying  evil  may  be 
cured.  A  rapid  doublet  may  be  excellent  for  groups, 
copying,  and  every  other  purpose,  and  yet  may  break 
down  when  employed  with  a  small  stop  in  landscape 
work.  This  class  of  flare-spot  is  seldom,  if  ever,  seen 
unless  a  small  stop  be  used. 

It  does  not  follow,  because  there  may  sometimes  be 
a  mistiness  or  haze  on  the  whole  or  a  portion  of  a  nega- 
tive, that  this  indicates  a  defect  in  the  lens.  We  have 
known  it  to  be  so  attributed  when  in  reality  it  was 
caused  by  the  admission  of  light  into  the  camera  through 
a  chink  almost  imperceptible  to  the  eye.  A  tiny  crack  in 
the  front  of  the  camera,  a  pinhole  in  the  bellows  body, 
the  absence  or  bad  fitting  of  a  screw  in  the  flange — 
these  and  other  causes  may  produce  deleterious  effects 
which  may  be  wrongly  attributed  to  the  lens. 


CHAPTER  XVII. 

THE   EQUIVALENT   FOCUS. 

PREMISING  that  the  solar  focus  of  a  lens  is  that  point 
at  which  objects  situated  at  a  great  distance  are  brought 
to  a  sharp  focus,  we  now  consider  the  nature  of  the 
'  equivalent '  focus  of  a  combination — a  term  which  arises 
from  a  comparison  with  a  single  lens  that  would  produce 
the  same-sized  image,  one  being  equivalent  to  the  other. 

What  is  the  Equivalent  Focus  ? — The  equivalent  focus 
of  a  lens  may  be  said  to  be  the  focus  measured  from  the 
optical  centre  of  the  combination  when  such  centre  has 
been  determined  for  a  distant  object.  The  term  'back 
focus,'  in  popular  use,  is  altogether  misleading,  or,  rather, 
it  conveys  no  idea  at  all  in  cases  in  which  accuracy  is 
required.  We  give  an  instance,  and  in  this  case  an 
extreme  one :  An  objective  may  be  formed  having  a 
back  focus  of  only  one  inch,  yet  the  real  or  equivalent 
focus  of  which  shall  be  eight  inches  ;  in  other  words,  the 
size  of  the  image  produced  by  the  combination  shall 
equal  that  produced  by  the  use  of  a  single  lens  of  eight 
inches  focus. 

Out  of  several  portrait  combinations  to  be  met  with 
every  day,  and  by  makers  of  high  reputation,  a  large 


EQ  UIVALENT  FOCUS.  101 

number  may  be  selected  almost  identical  as  regards  back 
focus,  but  not  two  alike  as  regards  real  or  equivalent 
focus.  We  were  present  in  the  establishment  of  a  dealer 
in  lenses  of  home  and  foreign  production  when  two 
portrait  lenses  were  selected  from  a.  large  stock  and 
accurately  paired,  as  was  imagined,  for  the  purpose  of 
being  employed  in  the  taking  of  instantaneous  stereo- 
scopic views.  Thorough  care  and  honesty  were  bestowed 
upon  the  selection,  the  mounts  were  identical  in  every 
respect,  and  both  were  then  brought  under  the  influence 
of  a  single  rack-and-pinion.  So  far  all  was  right,  and 
the  images  on  the  ground  glass  were  sharp.  Soon  after- 
wards they  were  returned  as  not  being  a  pair,  in  the 
sense  of  their  producing  images  of  different  dimensions. 
This  was  an  illustration  of  the  misleading  nature  of  back- 
focus  measurement.  It  being  of  importance  that  the 
photographer  should  know  the  real  focus  of  his  lenses, 
we  shall  now  give  some  methods  by  which  this  can  be 
ascertained. 

Rough  Method  of  ascertaining  the  Equivalent  Focus. — 
We  commence  by  giving  one  which  is,  at  frequent  inter- 
vals, being  discovered  by  some  whose  reading  of  photo- 
graphic literature  is  limited,  and  paraded,  especially  in 
non-photographic  serials,  with  all  the  trumpet-blowing 
of  a  great  discovery.  It  is,  unfortunately,  not  an  accurate 
method,  being  so  only  in  an  approximate  degree.  For 
'rough  and  ready'  purposes,  where  exactness  is  not 
essential,  it  may  prove  useful.  Focus  upon  any  subject 
— such  as  a  map  or  engraving — and  let  the  arrangement 
be  such  that  the  image  on  the  ground  glass  is  precisely 


102  GRUBB'S  METHOD. 

of  the  same  dimensions  as  the  original.  Now,  measure 
the  distance  between  the  ground  glass  and  the  subject, 
and  divide  by  four,  which  gives  the  figures  required. 
But,  as  we  have  said,  this  method  is  not  accurate  in  the 
case  of  a  combination  of  lenses. 

G-ruWs  Method. — Fortunately,  there  are  several  other 
methods  by  which  the  equivalent  focus  may  be  ascer- 
tained with  unfailing  accuracy,  and  in  describing  a  few 
of  them  we  commence  with  that  which  we  almost  in- 
variably employ  in  preference  to  all  others,  being  that  in 
which  the  late  Mr.  Thomas  Grubb  has  made  the  camera 
itself  to  do  duty  as  a  theodolite.  In  front  of  a  window 
place  a  table  covered  with  a  sheet  of  smooth  paper, 
which  must  be  fastened  to  the  table  top.  Now  make  a 
pencil  mark  at  each  side  of  the  ground  glass  of  the 
camera,  a  slight  distance  from  the  margin.  This  mark 
may  consist  of  a  line  about  an  inch  or  more  in  length. 
Next  direct  the  camera  to  any  well-defined  object  at  a 
distance — say,  the  top  of  a  chimney,  a  flag-staff,  the 
corner  of  a  building,  or  any  other  suitable  object — and 
rotate  the  camera  so  as  to  bring  this  object  directly  upon 
one  of  the  pencilled  lines  on  the  focussing-screen.  This 
having  been  done,  with  a  pencil  draw  a  line  on  the  paper 
cover  of  the  table,  making  use  of  the  right-hand  side 
base  of  the  camera  as  a  straight-edge  for  this  purpose. 
Now,  without  disturbing  the  table,  move  the  camera 
round  until  the  object  of  which  we  have  already  spoken 
is  brought  directly  upon  the  pencil  mark  at  the  opposite 
margin  of  the  focussing-screen,  and  again  draw  a  pencil 
line  on  the  sheet  of  paper,  using  the  right-hand  side  of 


P1NHOLE  METHOD.  103 

the  camera  for  this  purpose  as  before.  (We  may  here 
state,  par parenthhe,  that  the  two  lines  thus  drawn  show 
the  angle  of  view  included  within  the  space,  hence  this 
forms  a  simple  method  of  determining  the  angular  field 
given  by  any  lens.)  To  resume :  if  necessary,  extend 
the  lines  thus  projected  on  the  table  and  connect  them 
by  a  line,  as  in  the  cross  of  the  letter  A,  which  is  equal 
to  the  distance  apart  of  the  two  pencil  marks  on  the 
ground  glass.  The  distance  of  the  intersection  of  the 
first  two  lines  and  the  third  line  is  the  equivalent  focus 
of  the  lens. 

A  modification  of  the  system  described  consists  in 
determining  the  central  point  of  the  focussing-screen  by 
drawing  diagonals  from  the  corners.  Then  select  two 
distant  objects,  so  arranged  as  that  their  images  shall  be 
equidistant  from  the  central  point.  Measure  with  a  pair 
of  compasses  the  distance  between  the  two  objects  on 
the  ground  glass,  and,  rotating  the  camera  so  that  one 
of  them  shall  '  cut '  the  centre  mark,  draw  a  line  on  the 
sheet  of  paper  as  before  directed ;  then  turn  the  camera 
until  the  second  object  shall  in  like  manner  correspond 
with  the  central  mark,  a  second  line  being  drawn  on  the 
table.  Now  connect  these  two  angle  lines  by  a  third 
equal  to  the  space  between  the  compasses,  and  the  dis- 
tance between  the  junction  point  of  the  angle  lines  and 
the  cross  line  is  the  focus. 

The  Pinhole  Method. — Another  method  by  which  the 
equivalent  focus  of  a  combination  may  be  ascertained  is 
to  observe  very  carefully  the  size  of  the  image  of  any 
distant  object  given  upon  the  ground  glass,  then  remove 


104  SINGLE  LENS  METHOD. 

the  lenses  from  the  mount  and  insert — most  conveniently 
in  the  cell  for  the  stops — a  thin  plate  of  metal  in  which 
is  a  very  small  hole,  such  as  a  pinhole.  Now  move  the 
lens  mount  in  or  out  until  the  image  thus  obtained  coin- 
cides in  dimensions  with  that  given  by  the  lens  ;  then 
measure  the  distance  between  the  pinhole  and  the  ground 
glass.  This  will  be  practically  equal  to  the  equivalent 
focus  of  the  lens.  Owing  to  diffraction,  or  the  tendency 
of  rays  of  light  to  bend  when  passing  an  opaque  edge, 
it  will  be  impossible  to  secure  a  very  sharp  image  by 
this  pinhole  system.  On  this  we  may  observe  that 
although  in  geometric  optics  light  is  assumed  to  travel 
in  straight  lines  in  physical  optics  this  is  not  the  case, 
for  on  passing  by  the  edge  of  an  opaque  body  it  is  bent 
round  the  corner  to  some  small  extent. 

Single  Lens  Method. — Instead  of  the  pinhole  system  a 
better  way  is  to  obtain  a  cheap  biconvex  spectacle  glass, 
which  can  be  obtained  in  nearly  any  large  town  at  a  cost 
of  one  or  two  shillings  per  dozen.  Select  one  that  gives 
with  a  small  stop  an  image  the  same  size  as  the  com- 
bination. Measure  the  distance  between  the  centre  of 
the  glass  and  the  ground  glass,  although,  owing  to  the 
thinness  of  the  lens,  the  measurement  may  practically 
be  made  from  the  outer  surface.  Greater  accuracy  is, 
of  course,  secured  by  adding  to  the  measurement  thus 
obtained  the  semi-thickness  of  the  spectacle  lens. 

Rule-of -Three  Method. — But  it  is  not  at  all  necessary 
that  a  large  number  of  spectacle  glasses  be  obtained  for 
determining  the  equivalent  focus  of  a  combination,  seeing 
that  it  may  be  effected  by  the  use  of  one  alone  of  any 


RULE-OF-THREE  METHOD.  105 

known  focus.  Having  taken  the  precise  dimensions  of 
any  subject — and  which  we  may  designate  the  '  test 
object' — on  the  ground  glass  with  the  photographic 
combination  whose  focus  is  as  yet  unknown,  do  the  same 
with  the  spectacle  glass  of  known  focus,  and  compare 
the  two  results.  The  relation  of  the  sizes  of  the  two 
images  to  each  other  is  the  same  as  that  of  the  foci  of 
the  lenses  by  which  they  were  produced.  It  is  a  simple 
rule-of-three  problem. 

Several  other  methods  for  ascertaining  the  equivalent 
or  solar  focus  have  been  suggested,  but  those  here  given 
will  serve  every  purpose,  and  may  be  practised  very 
simply.  Hence  to  avoid  complications  we  confine  our- 
selves to  tnern. 


CHAPTER    XVIII. 

CONJUGATE  FOCI. 

IF  a  lens  which  has  been  carefully  focussed  upon  a 
distant  object  be  then  directed  towards  one  compara- 
tively near  at  hand,  the  nearer  object  will  be  found  to  be 
out  of  focus,  necessitating  the  withdrawal  of  the  ground 
glass  from  the  lens  before  the  image  will  assume  its 
maximum  sharpness.  This  establishes  the  fact  that 
there  exists  a  relation  between  the  object  that  is  focussed, 
as  regards  its  distance  from  the  camera,  and  the  focus  of 
the  lens.  This  relation  is  termed  *  conjugate  foci'  In 
what  we  have  now  to  say  we  will  speak  of  the  distance 
between  the  lens  and  the  object  as  the  anterior  or  major 
conjugate,  and  that  existing  between  the  lens  and  the 
ground  glass  of  the  camera  as  the  posterior  or  minor 
conjugate  focus. 

Conjugate  Focus  Illustrated. — Parallel  rays  a  a — that 
is,  rays  from  a  great  distance — falling  upon  a  lens  come 
to  a  focus  at  f;  but  those  from  b,  which  may  serve  to 
represent  any  object  ten  or  twenty  yards  distant,  have 
their  focus  at  c  (Fig.  41).  f  is  the  solar  focus,  b  and 
c  are  conjugate  foci,  and  the  former  of  these  is  the 
anterior,  and  the  latter  the  posterior  conjugate.  To 


LAWS  GOVERNING  CONJUGATE  FOCI.  107 

facilitate   reference,  the   lines   indicating  the  conjugate 
foci  are  solid,  while  those  relating  to  the  solar  focus  are 


>c 


FIG.  44. 

dotted.  The  points  b  and  c  are  interchangeable ;  an 
object  placed  at  either  is  sharp  at  the  other. 

Laws  governing  Conjugate  Foci. — The  laws  which  govern 
the  conjugate  foci  are  to  be  found — not,  perhaps,  so 
clearly  expressed  as  the  practical  photographer  would 
require — in  several  old  optical  treatises.  The  following, 
which  amount  to  nearly  the  same  thing,  although  ex- 
pressed differently,  will  be  quite  sufficient  for  introduction 
in  this  chapter  : — 

Claudet's  Rule  for  estimating  Conjugate  Foci — If  the 
principal  or  solar  focus  of  a  lens  be  regarded  as  the  unit 
of  measure,  an  object  situated  in  front  of  the  lens  at  a 
distance  from  a  certain  point,  equivalent  to  a  multiple 
of  the  said  unit,  will  have  its  conjugate  posterior  focus 
at  a  distance  from  another  certain  point  equal  to  a 
corresponding  fraction  of  the  same  unit  This  relation 
of  the  conjugates  to  each  other,  although  probably  first 
published  in  aa  old  work  (Dr.  Smith's  Optics),  was  first 
brought  before  the  world  in  relation  to  photography  by 
the  late  M.  A.  Claudet  at  the  Aberdeen  meeting  of  the 


icS  BREWSTER'S  RULE. 

British  Association  (1859).  The  following  popular  illus- 
tration, which  was  given  at  the  time  of  the  first  publication 
of  the  proposition  in  The  British  Journal  of  Photography, 
serves  to  make  it  more  readily  understood  : — Suppose 
we  have  a  lens  of  twelve  inches  solar  focus — an  object 
situated  at  a  distance  of  six  feet  from  a  certain  point  in 
front  of  the  lens— that  is,  at  six  times  the  unit  of  measure 
— will  have  its  conjugate  posterior  focus  at  a  distance  of 
one-sixth  part  of  the  same  unit — that  is,  at  two  inches 
distance — from  a  corresponding  point  behind  the  lens. 

The  '  point '  here  spoken  of  before  or  behind  the  lens 
is  the  solar  focus  measured  from  the  optical  centre  of 
the  combination,  or,  as  we  described  it  in  the  previous 
chapter,  the  centre  of  conjugate  foci. 

Brewster's  Riile. — Previous  to  the  publication  of  this, 
one  of  the  methods  usually  adopted  to  calculate  the 
conjugate  foci  was  that  of  Sir  David  Brewster,  which, 
however,  was  of  little  or  no  use  when  applied  to  other 
than  a  simple  lens  :— Multiply  twice  the  product  of  the 
radii  of  the  two  surfaces  of  the  lens  by  the  distance  of 
the  radiant  point  from  the  centre  of  the  lens  for  a  divi- 
dend. Multiply  the  sum  of  the  two  radii  by  the  same 
distance,  and  from  this  product  subtract  twice  the  pro- 
duct of  the  radii  for  a  divisor.  Divide  the  above  dividend 
by  the  divisor,  and  the  quotient  will  be  the  focal  distance 
required. 

From  what  was  said  in  the  previous  chapter,  it  will 
be  understood  that  the  range  of  posterior  conjugate  focus 
extends  only  from  the  solar  focus,  which  is  the  nearest 
point  to  the  lens  at  which  a  focus  of  any  kind  can  be 


GRUBffS  METHOD.  ^09 

obtained,  and  that  focus  which  results  from  having  the 
object  so  near  to  the  lens  as  to  give  an  image  of  the 
same  dimensions  as  the  object,  and  which,  as  we  have 
shown,  is  twice  the  solar  focus. 

Grubb's  Method  and  Table. — Soon  after  the  publication 
of  M.  Claudet's  method,  as  just  described,  the  late  Mr. 
Thomas  Grubb  directed  his  attention  to  the  proposition 
with  a  view  to  its  still  further  simplification  and  per- 
fecting for  photographers'  use.  We  here  present  two 
tables  in  juxtaposition — No.  T  containing  four  ratios  con- 
structed in  accordance  with  M.  Claudet's  method;  No.  2 
being  based  upon  the  shortcoming  of  the  other,  in  which 
there  is  nothing  to  indicate  any  ratio  required  except 
that  of  I  to  i,  and  in  which  (viz.,  in  No.  2)  Mr.  Grubb 
adopts  in  preference  the  more  simple  and  natural  ratios 
ot  the  actual  distances  from  the  lens. 

No.  i.  No.  2. 

1  /and  i  /  2  /and  2  / 

2  /and  i/  ...         3 /and  |/ 
3 /and  i/  4 /and  |/ 
4/andi/  5 /and  |/ 

In  table  No.  2  the  proportions  required  are  at  once 
apparent.  The  numbers  denote  the  actual  distances 
required  to  be  used  for  a  focus  of  one  foot,  and  the  ratio 
is  still  of  so  simple  a  progressive  nature  that  a  table 
of  any  required  extent  may  be  constructed  almost  as 
quickly  as  the  figures  can  be  written. 


no  GRUB&S  METHOD. 

Having  given  Mr.  Grubb's  table  (No.  2),  we  here 
present  in  a  condensed  form  his  argument  based  upon 
it,  and  the  simple  arithmetical  rule  deducible  therefrom, 
by  which  to  determine  the  conjugates  :— 

Let  it  be  borne  in  mind,  first,  that  /  represents  the 
focus  of  the  lens,  and  that  this  focus  is  assumed  to  be 
=  i  foot,  or  unity  ;  and,  secondly,  that  we  do  not  alter 
the  poiver  of  a  lens  by  using  it,  whether  for  bringing 
parallel  rays  to  a  focus  or  for  forming  conjugate  foci. 
What  we  do  in  the  latter  case  is  simply  to  use  a  portion 
of  its  power  on  one  side,  leaving  the  balance  of  its 
power  to  be  exerted  on  the  other  side — the  simplest  case 
of  this  being  that  where  we  use  the  lens  for  forming 
equal  conjugate  foci,  and  where,  the  lens  being  one  foot 
in  principal  focus,  a  power  equivalent  to  a  focus  of  two 
feet  is  used  at  one  side,  leaving  an  equal  power  to  be 
exerted  at  the  other  side.  Now  it  requires  very  little 
mathematical  knowledge  to  perceive  that  we  can  only 
perform  the  operation  of  adding  and  subtracting  such 
powers  by  treating  them  as  fractions — that  is,  by  using 
their  reciprocals  ;  and  thus,  as  we  express  the  adding  of 
two  halfpennies,  namely, 

\  +  |  =  i  =  i  penny, 

we  in  like  manner  must,  in  adding  the  two  before- 
mentioned  of  two  feet  each  in  focus,  adapt  the  formula 
(p  and  /"l  being  put  for  the  respective  powers)  : — 

\  +  £  =  y  (and  /  and  p1  being  each  =  2  feet). 

%  4-  J  =  Y  or  focus  =  i. 

From  this  simple  equation  (calling  the  whole  power  of 
the  lens  I,  or  unity)  we  gather  that  the  sum  of  the 


GRUB&S  METHOb.  Hi 

reciprocals  of  the  powers,  which  are  at  the  same  time 
the  required  distances  from  the  lens,  must  equal  unity  ; 
that  is,  any  two  fractions  whose  sum  is  unity  will,  in 
their  reciprocals,  give  relative  distances  of  the  object 
and  image  for  a  lens  whose  principal  focus  is  I — foot, 
yard,  &c. 

The  rule  deducible  from  the  foregoing  for  finding 
the  required  distance  for  any  proportional  size  of  object 
and  image,  and  for  any  given  focus  of  lens,  is  :  Add 
the  required  proportions  together  for  the  denominator 
of  two  fractions  whose  numerators  are  the  separate 
numbers.  Invert  these  fractions,  and  multiply  the  focus 
of  the  lens  by  each  of  these  for  the  respective  distance. 


CHAPTER    XTX. 

THE  PRINCIPLE  OF  CONJUGATE  FOCI  APPLIED  TO  HAND 
CAMERAS  AND  FOR  ENLARGEMENT. 

Hand  Cameras.  —  One  practical  application  of  the 
principle  of  conjugate  focus  is  the  construction  of  scales 
of  distance  for  hand-cameras,  rendering  the  focussing  of 
each  object  unnecessary.  Every  photographer  is  now 
aware  that,  if  he  focus  a  distant  object  very  sharply,  and 
then  make  a  mark  on  the  adjusting  portion  of  his 
camera,  no  re-focussing  will  ever  afterwards  be  required 
when  taking  a  distant  object,  all  that  is  necessary  being 
to  slide  out  the  camera  until  the  previously  made 
adjustment  marks  coincide.  In  like  manner  adjust- 
ment marks  may  be  made  for  objects  situated  at 
shorter  distances.  The  value  of  this  will  be  specially 
appreciated  under  a  twofold  class  of  circumstances, 
namely,  when  by  accident  the  focussing-glass  gets 
broken ;  but  more  especially  when  the  object  to  be 
photographed  is  in  motion,  precluding  the  possibility  of 


ENLARGING  AND  REDUCING.  113 

staying  in  order  to  have  it  focussed,  To  focus  ships  in 
motion,  especially  from  the  deck  of  another  ship  also  in 
motion,  is  altogether  out  of  the  question  when  the  whole 
powers  of  the  photographer  are  taxed  in  observing 
the  fitting  moment  at  which  to  touch  the  exposing 
trigger.  In  such  a  case  the  proper  procedure  is  to 
estimate  as  nearly  as  possible  the  distance  at  which  the 
ship  is  from  the  lens  (the  acquisition  of  such  guessing 
power  being  by  no  means  difficult),  and  then  adjust 
the  sliding  portion  of  the  camera  or  lens  to  the  corre- 
sponding mark. 

Enlarging  and  Reducing. — It  is,  however,  in  the  pro- 
duction of  enlargements  and  enlarging  requirements, 
together  with  those  employed  in  copying  of  every 
description,  that  the  use  of  a  knowledge  of  the  laws 
of  conjugate  foci  will  be  exceptionally  useful.  A  photo- 
grapher is  supposed  to  be  desirous  of  knowing  what 
dimensions,  as  regards  length,  he  should  adopt  in  con- 
structing a  camera  in  which  he  will  be  able  to  copy  a 
picture  or  object  several  times  larger  or  smaller  than  the 
original,  and  to  know  how  far  from  the  lens  must  be 
the  object  on  the  one  hand  and  the  ground  glass  on 
the  other.  He  is  further  supposed  to  have  two  or 
three  lenses  of  different  foci,  but  of  the  precise  equi- 
valent focus  of  each  of  which  he  has  made  himself  well 
aware  by  one  of  the  methods  described  in  our  last 
chapter. 

Now  let  that  focus— whether  five,  six,  eight,  or 
nine  inches — be  represented  by  /  This  is  the  only 


ii4  TABLE  OF  VTEW  ANGLES. 

known  element  in  the  inquiry  at  the  present  stage. 
What  is  now  required  are  the  conjugates  at  which 
to  place  the  negative  to  be  enlarged  (represented  by 
n)  and  the  focussing  -glass  respectively,  so  that  a 
sharp  image  shall  be  produced,  no  matter  what  may 
be  the  degree  of  enlarging.  Expressing  one  focus 
of  the  lens  by  u  and  the  other  by  v  we  have  the 
following  :  — 

(1)  u  —  (n-\-  i)/  and 

(2)  V= 


which,  when  converted  into  simple  language,  means  — 

(1)  Add  one  to  the  times  of  enlargement  (or  reduc- 
tion) desired,  and  multiply  the  sum  by  the  equivalent 
focus  of  the  lens.      The  product  is  the  length   sought 
for. 

(2)  To  find  the  other  conjugate  focus  :    Divide  the 
equivalent    focal    length    of  the    lens    by  the    times    of 
enlargement  (or  reduction)   required,  and  add  it  to  the 
equivalent  focal  length.     The  sum  is  the  length  sought 
for. 

The  above  embraces  the  whole  subject  of  enlarge- 
ment and  reduction,  even  though  the  degree  of  en- 
larging be  such  as  extends  to  the  production  of  a  life-size 
picture  from  a  small  miniature. 

Table  of  View  Angles.  —  The  following  useful  table, 
calculated  by  Dr.  C.  E.  Woodman,  of  New  York,  was 
published  in  the  Photographic  Times  during  its  editor- 
ship by  the  author. 


WOODMAN* S  TABLE  OF  VIEW  ANGLES.        115 


DIVIDE  THE  BASE*  OF  THE  PLATE  BY  THE  EQUIVALENT  Focus  OF 
THE  LENS. 


If  the  quo- 
tient is 

The 

angle  is 

If  the  quo- 
tient is 

The 
angle  is 

If  the  quo- 
tient is 

The 
angle  is 

Deg?-ees. 

Degrees. 

Degrees. 

•282 

16 

748 

41 

i  '3 

66 

'3 

17 

768 

42 

1-32 

67 

•317 

18 

•788 

43 

68 

]335 

19 

•808 

44 

1-375 

69 

20 

•828 

45 

i'4 

70 

'37J 

21 

•849 

46 

1-427 

•389 

22 

•87 

47 

i'45 

72 

•407 

23 

•89 

48 

1-48 

73 

•425 

24 

•911 

49 

i  '5 

74 

'443 

25 

'933 

50 

i  -53 

75 

•462 

26 

•954 

51 

1-55 

76 

•48 

27 

'975 

52 

i  '59 

77 

;s 

28 

i* 

53 

1-62 

78 

29 

1*02 

54 

1-649 

79 

•536  . 

30 

•041 

55 

1-678 

80 

•555 

31 

'063 

56 

17 

81 

•573 

32 

•086 

57 

1-730 

82 

•592 

33 

•108 

58 

1769 

83 

•611 

34 

"I  *2 

59 

•8 

84 

•631 

35      ; 

•155 

60 

•833 

•65 

36 

•173 

61 

•865 

86 

•67 

37 

•2 

62 

•898 

87 

•689 

38 

•225 

63 

•93* 

88 

7c8 

39 

•25 

64 

•965 

89 

728 

40 

•274 

65 

2' 

90 

Example. — Given  a  lens  of  13  inches  equivalent  focus  ;  required  the 
angle  included  by  it  on  plates  respectively  3^  x  4^,  4^  x  6],  6£  x  8^,  8  x  10, 
lox  12,  and  II  x  14. 

(i)  Dividing  4-25  by  13,  we  have  as.  quotient  -327 — midway  between 
the  decimals  '317  and  '335  of  our  table;  therefore  the  required  angle  is 
18"  30'.  Similarly 

Degrees. 

13         =  '5  :        corresponding  to  28. 

13          =  '654;  „  „   36. 

o  77  ;  „  „   42i 

'3         =  "923:  ,»  „  49^. 

13         =         i 'OS;  „  „  57- 


',2) 
(3) 
(4) 
(5}  I2 
(6)  14 


6-5 
S-5 


*  Th's  is  rot  strictly  accurate,  but  if  the  dhftonal  of  the  plate  be  substituted  for  the  base,  the  angle 
found  will  be  correct,  if  the  lens  be  placed  opposite  the  centre  of  the  plate. 


CHAPTER  XX. 

A  MECHANICAL  MEANS  OF  ESTIMATING  CONJUGATE 
FOCI. 

IN  the  previous  chapter  the  means  for  ascertaining 
conjugate  foci  involve  a  certain  amount  of  calculation, 
although  not  much. 

Sir  Howard  Grubb's  System. — But  for  the  numerous 
class  of  photographers  who  dislike  mathematical  cal- 
culations, a  method  has  been  devised  by  Sir  Howard 
Grubb,  F.R.S.,  a  method  so  simple  and  withal  so  accurate 
as  to  have  elicited  the  highest  encomiums  from  those 
competent  to  form  an  opinion.  We  give  it  in  Sir 
Howard's  own  language. 

Draw  on  a  board,  wall,  or  floor,  a  square  A  B  C  D, 
each  side  of  which  is  equal  to  the  focus  of  the  lens  ; 
produce  two  adjacent  sides  of  the  square  C  B  and  c  D. 
At  A  insert  a  pin  or  nail.  Now  place  a  rule  or  straight 
edge  and  rocking  it  on  the  pin  or  nail  there  inserted, 
observe  where  it  cuts  the  prolonged  sides  of  the  square, 
as  at  M  and  N  or  M'  and  N'. 

No  matter  what  position  you  place  the  rule  in 
(always  provided  it  rests  against  the  pin  at  A  and 
cuts  the  prolonged  sides  of  square),  the  distances  C  M 


HOWARD  GRUBB'S  SYSTEM. 


117 


and  C  N  will  represent  a  pair  of  conjugates  for  that 
particular  lens.  If  it  be  required  to  enlarge  or  diminish 
by  four,  six,  or  any  definite  number  of  times,  it  is  only 
necessary  to  rock  the  rale  on  the  pin  till  one  of  the 
distances  C  M  is  four  or  six  times  more  or  less  than  the 


FIG.  45- 

other  C  N.  In  other  words,  a  lens  of  any  focus  equal  to 
C  B  will  form  an  image  of  an  object  placed  at  a  distance 
of  C  N  at  the  points  C  M,  £c. 

Similarly,  if  the  focus  of  the  lens  be  not  known,  but 
that  the  distance  is  known  at  which  an  image  is  formed 
behind  lens  of  any  object  at  a  known  distance  in  front 
of  same,  and  that  it  is  desired  to  know  the  focus  of  that 
lens  :  measure  off  the  distance  of  the  object  from  lens 


ii8  CONJUGATE  FOCt. 

on  a  horizontal  line  as  at  C  N  and  the  distance  of  imag 
from  lens  on  a  vertical  line  as  at  C  M,  lay  straight-edge 
across  them  and  observe  where  this  cuts  the  diagonal 
line  as  at  A,  then  draw  A  B   parallel  to  horizontal  line, 
and  C  B  or  A  B  is  the  solar  focus  of  lens. 

The  above,  which  may  prove  useful  to  those  engaged 
in  enlarging  operations,  depends  upon  the  fact  that  in 
the  figure  given  : — 

I          I          i  C  M  +  c  N 

C~M  +  C~N  =  cTe  or  c     =  cTi "+C~N 

Now  as  this  addition  and  subtraction  of  reciprocals 
enters  very  largely  into  many  optical  calculations,  it 
will  be  seen  that  the  above  is  only  one  of  many  cases 
in  which  this  graphical  method  may  be  utilised. 

Immediately  after  Sir  Howard  sent  us  the  account 
of  this  system  for  publication  we  lost  no  time  in  having 
it  constructed,  which  we  did  by  fixing  on  a  thin  slab  of 
wood  two  ordinary  rules  graduated  to  feet  and  inches, 
one  placed  vertically  as  at  C  M,  the  other  horizontally  as 
C  N.  The  diagonal  line  C  A  was  a  slot  in  which  travelled 
a  pin  or  stud  with  a  pinching  screw  behind,  by  which 
it  was  capable  of  being  adjusted  to  suit  the  focus  of  any 
lens,  the  distance  between  the  stud  and  vertical  or 
horizontal  rules  equalling  the  focus  of  the  lens. 

A  piece  of  apparatus  of  this  kind,  which  every  one 
can  make  for  himself  at  a  very  trifling  expenditure  of 
money  or  labour,  is  a  thing  which  we  can  strongly 
recommend  to  all  who  have  to  do  copying  or  enlarging, 
as  the  major  and  minor  conjugates  of  the  lens  —  the 
positions  respectively  of  the  negative  and  the  sensitive 


THE  CAMERA  CI.UB  FOCIMETEK. 


119 


surface  —  can  be  ascertained  at  a  moment  for  any  given 
degree  of  enlargement  or  reduction. 

The  Camera  Club  Focimeter. — When  devising  a  foci- 
meler  for  the  use  of  the  Camera  Club,  Mr.  Lyonel 
Clark,  C.E.,  selected  as  a  basis  that  of  Sir  Howard 
Grubb,  just  described,  to  which  he  made  some  additions, 
so  as  to  render  it  applicable  for  any  establishment  where 
enlarging  on  a  large  and  varied  scale  is  carried  on.  For 
the  following  drawing  and  description  we  are  indebted 
to  Mr.  Clark. 


FIG.  46. 

This  apparatus  is  constructed  for  lenses  of  any  focal 
length,  but  for  amateurs  who  only  use  a  lens  of  one 
focal  length  it  can  be  made  in  a  more  simple  form. 


120  CONJUGATE  FOCt. 

To  construct  the  simpler  form  of  apparatus  you  lay 
off  a  right  angle,  BAG,  and  divide  its  two  sides,  A  B, 
A  C,  into  feet  and  inches ;  the  length  of  A  B,  which 
represents  the  major  conjugate,  is  of  course  determined 
by  the  extension  of  the  enlarging  camera  or  ease).  Not 
to  have  too  bulky  an  apparatus,  the  sides  will  best  be 
divided  to  some  scale,  say  one-quarter  or  one-eighth. 

You  next  divide  the  right  angle  into  two  equal  parts 
by  the  diagonal  A  D  ;  to  obtain  the  correct  position  of 
the  pin  P,  on  which  the  straight-edge  rocks,  you  have  to 
erect  a  perpendicular  on  either  side  at  the  division  on 
the  scale  corresponding  to  the  focal  length  of  the  lens 
to  be  used. 

In  the  cut  the  slide  is  set  for  a  10"  lens,  and  there- 
fore the  perpendicular,  O  P,  is  erected  at  the  10"  mark 
on  A  C,  and  the  spot,  P,  where  O  P  cuts  the  diagonal, 
A  D,  is  where  the  pin  has  to  be  placed.  Against  this 
pin  any  ordinary  straight-edge  is  rocked.  It  is,  of  course, 
best  to  let  a  small  piece  of  brass  into  the  straight-edge, 
through  which  the  pin  is  inserted;  this  prevents  shifting. 
The  straight-edge  is  furnished  with  an  index,  or  pointer, 
P  P'.  This  is  best  placed,  for  the  sake  of  symmetry, 
not  at  right  angles  to  the  scale,  but  22  J°  less. 

For  a  large  establishment,  where  lenses  of  different 
foci  arc  used,  the  straight-edge,  with  its  pivot,  pointer, 
and  quadrant,  are  carried  on  a  moving  piece  and  can 
slide  up  and  down  the  diagonal  A  D,  which  is  now 
divided  off  in  a  continuous  scale  of  foci.  These,  of 
course,  are  obtained  in  the  same  manner  as  the  single 
focus  was  obtained. 


CONJUGATE  FOCI.  121 

The  manner  of  graduation  is  done  by  calculating  out 
a  series  of  diameters  of  enlargements  for  one  known 
lens.  We  need  only  deal  with  one  conjugate,  preferably 
the  major,  A  C.  The  equation  for  this  length  is  (n+  i)fy 
where  n  =  number  of  times  of  enlargement,  and  /  the 
focal  length  of  the  lens.  Now  we  can  take  the  focal 
length  of  our  lens  as  anything,  we  will  make  it  unity  in 
inches,  and  the  equation  becomes  n  +  i  ;  that  is,  the 
length  of  the  major  conjugate  is  the  number  of  diameters 
of  enlargement  plus  one  (expressed  in  inches).  To 
enlarge  one  diameter — that  is,  to  obtain  an  image  of 
equal  size  —  it  is  I  + 1,  that  is,  2  ;  for  2  diam.  3  ;  for  3 
diam.  4 ;  and  so  on.  As  one  inch  is  so  small  a  thing 
to  deal  with,  it  is  best  to  take  10"  as  the  focus  of  the 
lens.  This  only  alters  the  decimal  point ;  2  diam.  is 
still  30  inches,  and  has  the  advantage  that  each  added 
inch  represents  a  tenth  of  a  diameter.  So  practically, 
setting  our  index  (thefaur  de  lys)  at  10",  we  swing  the 
straight-edge  until  it  cuts  the  20"  mark  on  A  C,  and 
there  mark  the  spot  at  which  the  pointer,  P  P',  stands 
as  I  diam.  ;  moving  the  straight-edge  to  21"  we  mark  off 
from  the  pointer  ri  diam.,  at  22"=  1*2  diam.,  at  23"^= 
1*3  diam.,  and  so  on  for  each  succeeding  inch. 

The  scale  of  diameters  of  enlargement  thus  laid  out 
is  true  for  whatever  lens  we  like  to  adjust  our  slide  to. 
Whatever  focus  we  arc  using,  if  we  set  the  fleur  de  lys 
to  it,  and  then  swing  our  pointer,  P  P',  to  the  number  of 
diameters  we  wish  to  enlarge,  we  shall  read  off  where 
the  straight-edge  cuts,  A  B  and  A  C,  the  length  of  the 
two  conjugates. 


122  CONJUGATE  FOCI. 

In  the  cut  thefaur  de  lys  is  set  for  a  10"  lens,  and 
the  pointer  indicates  1*65  diameters.  We  read  off  on 
the  major  conjugate,  A  B  —  that  is,  the  distance  from  the 
lens  centre  to  the  enlargement  —  2'*2  J",  and  on  the  minor, 
that  is,  the  distance  from  the  lens  to  the  negative,  i''4$". 

Let  us  check  this  by  calculation. 

The  major  conjugate  =          (n+i)  f 

(1-65  +  1)  10" 


major  conjugate 
minor  conjugate  =    - 

n 
26-5  .     . 

m  inches- 


1  6"'  i 


The  accuracy  of  result  must,  of  course,  depend  on 
the  care  in  the  manufacture  of  the  instrument 

It  is,  perhaps,  hardly  necessary  to  point  out  that 
in  the  case  of  reduction  the  figures  remain  the  same,  but 
the  major  axis  is  now  the  distance  of  the  negative  from 
the  lens,  and  the  minor  axis  the  distance  of  the  lens 
from  the  reduction. 


CHAPTER  XXI. 

DEPTH   OF   FOCUS. 

A  Paradox. — In  discussing  this  subject,  we  begin  some- 
what paradoxically  by  stating  that  there  is  no  such  thing 
as  depth  of  focus.  Optically  speaking,  the  focus  of  a 
lens  is  a  point ;  and  in  cases  where,  from  aberrations, 
the  rays  from  any  object  do  not  converge  to  a  point, 
of  such  a  lens  it  may  then  be  said  that  it  possesses  no 
true  focus  at  all. 

But,  it  may  be  asked,  How  does  it  happen  that  if 
an  object  at  a  reasonable  distance — say,  a  quarter  of  a 
mile  —  be  sharply  focussed,  all  objects  beyond  that  will 
also  be  sharp  ?  To  meet  this  we  say  that  if  a  lens  of 
long  focus  capable  of  yielding  a  sharp  image  be  em- 
ployed, this  will  not  be  found  to  be  the  case.  If  the 
focus  of  an  object  at  the  distance  of  a  mile  be  carefully 
found  in  a  telescope  of,  say,  eight  inches  aperture  and 
proportionate  focus,  another  object  situated  a  quarter  of 
a  mile  away  from  the  former  will  be  quite  out  of  focus. 

Brewster,  in  his  Treatise  on  New  Philosophical 
Instruments,  shows  that  he  was  quite  aware  of  this 
property  in  lenses,  for  he  gives  instructions  how,  by 
means  of  a  graduated  eye-tube,  a  telescope  may  be 


124  NATURE  OF  DEFINITION. 

constructed  which  shall,  within  certain  limits,  show  the 
distance  at  which  any  object  is  from  the  observer. 
Were  there  such  a  property  as  depth  of  focus,  it  is 
evident  that  such  a  telescope  could  not  be  constructed. 

The  Nature  of  the  Definition  required  in  Photography. — 
But  the  image  produced  by  means  of  a  photographic 
lens  is  of  a  different  quality  so  far  as  concerns  sharpness 
from  that  formed  by  either  a  telescopic  or  microscopic 
object-glass,  for  the  conditions  required  to  be  fulfilled 
by  the  former  differ  from  the  others.  The  sharpest 
possible  definition  of  objects  situated  in  various  planes 
of  distance  —  this  definition  not  being  confined  to  a 
limited  spot  in  the  axis  of  the  object-glass  as  in  the 
telescope,  but  spread  over  a  field  of  considerable  width 
— is  required  in  the  photographic  lens. 

A  lens  fulfilling  the  requirements  of  the  photographer 
should  not  have  a  mathematical  focus  or  a  definite  focal 
point,  but  should  possess  such  a  degree  of  aberration  as 
to  yield,  with  a  moderate  aperture,  good  pictorial  sharp- 
ness of  objects  in  various  planes.  We  possess  a  whole- 
plate  portrait  lens,  four  inches  in  diameter,  in  which 
there  was  so  little  depth  of  definition  that  in  taking  a 
portrait  when  the  tip  of  the  nose  was  sharp,  the  eyes 
and  mouth  were  quite  out  of  focus.  Of  course  we  could* 
by  the  insertion  of  a  small  diaphragm,  bring  both  into 
equal  apparent  sharpness,  but  this  entailed  a  prolonged 
exposure.  But  by  destroying  the  optical  perfection  of 
focus  which  characterised  this  lens,  we  have  now  obtained 
such  a  balance  of  advantages,  that  with  a  wide  aperture 
we  have  still  pictorially  good  definition  of  the  various 


CONTRACTING  THE  APERTURE. 


125 


planes  of  the  face  and  body,  and  a  more  photographically 
useful,  although,  optically,  a  less  perfect  instrument,  is 
the  result  of  the  alteration. 

Depth  of  focus,  or,  more  correctly,  of  definition,  is 
increased  by  the  employment  of  a  smaller  aperture. 
By  one  of  the  diagrams  in  the  second  chapter  we  have 
shown  the  effect  of  a  stop  in  producing  sharpness  by 
shutting  out  rays  which  would  confuse.  In  the  following 
figure  we  show  the  influence  of  the  stop  in  extending 
the  range  of  focus.  With  full  aperture  as  indicated  by 


FIG.  47. 

the  two  outside  converging  lines,  the  focus  is  at  a  definite 
point,  the  slightest  removal  of  the  sensitive  plate  from 
which  would  impair  the  definition.  But  suppose  a 
diaphragm  is  inserted  which  admits  only  the  acute 
angle  of  rays  at  the  centre,  then  will  it  be  seen  to  what 
extent  the  focal  plane  may  be  varied  from  the  true  focal 
point,  without  very  seriously  impairing  the  definition. 

Objects  served  by  Contracting  the  Aperture. — In  a  land- 
scape lens,  or,  for  that  matter,  in  many  other  lenses, 
the  contraction  of  the  aperture  by  a  stop  serves  the 
threefold  purpose  of  enhancing  definition  by  diminution 
of  spherical  aberration;  depth  of  focus  by  causing  the 


126  FIXED  FOCUS  LENSES. 

converging  pencil  of  rays  to  fall  on  the  plate  at  a  more 
acute  angle ;  and  flatness  of  field  by  extending  the 
oblique  pencils.  When  a  stop  is  employed  with  a 
landscape  lens,  the  focus  received  on  the  plate  is  not 
a  mathematical  intersection  of  lines  forming  a  point, 
but  is  composed,  so  to  speak,  of  a  cylinder  which  can  be 
cut  at  varying  distances  from  the  lens,  within  certain 
limits,  without  greatly  impairing  the  definition. 

Fixed  Focus  Lenses  for  Landscape  Work. —  Previous  to 
the  advent  of  the  detective  or  hand  camera,  since  which 
this  has  been  better  understood,  the  question  has  been 
frequently  raised  as  to  the  expediency  of  having  a  rigid 
camera  with  a  fixed  lens  for  landscape  work.  The 
principle  of  depth  of  focus,  or  penetration,  enables  this 
to  be  successfully  accomplished,  for  when  the  lens  is 
focussed  on  distant  objects,  it  is  found  that  everything 
desired  to  be  included  in  a  view  will  be  well  defined. 
The  shorter  the  focus  of  the  lens,  the  greater  is  the  depth 
of  definition,  so  that  in  the  case  of  two  lenses  —  one  long 
and  the  other  short  in  focus  —  which  are  focussed  on 
distant  objects,  the  latter  will  include  a  greater  range 
of  sharply-defined  objects  in  the  foreground  than  the 
former.  We  have  seen  it  laid  down  as  an  approximative 
rule  by  some  writer  on  optics  ^Thomas  Sutton,  if  we 
remember  aright),  that  if  the  diameter  of  the  stop  be  a 
fortieth  part  of  the  focus  of  the  lens,  the  depth  of  focus 
will  range  between  infinity  and  a  distance  equal  to  four 
times  as  many  feet  as  there  are  inches  in  the  focus  of 
the  lens.  Assuming  this  to  be  correct,  let  us  suppose 
that  an  operator  in  the  field  has  two  cameras,  one  with 


TABLE  OF  FOCI. 


127 


a  lens  of  four  and  the  other  of  fifteen  inches  focus  ;  in 
taking  views  with  these  from  the  same  spot,  the  nearest 
objects  which  in  the  case  of  the  larger  instrument  can 
be  introduced  will  be  at  a  distance  of  sixty  feet,  while 
with  the  smaller  camera  objects  situated  sixteen  feet 
distant  will  be  included  with  equal  sharpness. 

A  few  years  ago  a  Committee  of  the  Society  of 
New  York  Amateurs  was  appointed  to  compile  a  table 
showing  the  range  of  focus  for  detective  camera  lenses. 
The  following  is  the  result.  It  shows  the  number  of 
feet  beyond  which  everything  is  in  focus  when  the 
equivalent  focus  indicated  is  used.  The  disc  of  con- 
fusion is  less  than  one  hundredth  of  an  inch. 


Equivalent 
focus  lengths. 

Stop. 
5~ 

Stop. 

10 

15 

20 

25 

3° 

i 

f 

50 

60 

2  inches 

ioi 

ll 

i 

2 

3 

1 

23 

'i 

2 

i 

4 

I 

3 

i52 

8 

5 

4 

3 

3 

2" 

J4 

i^ 

4 

27 

14 

9 

7 

si 

5 

3? 

3 

22 

5 

46 

21 

14 

ii 

9 

7* 

6 

ti 

4 

6 

60 

3° 

20 

15^ 

12} 

ioi 

8 

4 

sl 

7 

82 

42 

27 

21 

17 

ipj 

9 

8 

107 

54 

36 

27 

22 

19 

14 

ii 

TO 

c, 

137 

68 

45  !  ^4.  !  28 

23 

18 

14 

12 

IO 

167 

84 

5° 

42    34 

30 

21 

18 

15 

1  1 

202   |  101 

67 

5< 

41 

37 

26 

21 

18 

12 

211 

T2I 

8c 

61 

49 

4* 

31 

25 

21 

1  } 

14 

283 
328 

142 
I64 

109 

71 
83 

I 

48 
56 

37 
42 

34 

25 
29 

376 

I89 

125 

95 

76 

64 

48 

39 

33 

CHAPTER   XXII. 

DIFFUSION   OF   FOCUS. 

Meaning  of  'Diffusion.' — The  term  '  diffusion  of  focus* 
is  another  name  for  spherical  aberration.  Some  imagine 
that  portrait  lenses  possessing  this  property  have  an 
advantage,  not  shared  by  others,  of  equalising  the  de- 
finition of  varying  planes  ;  this,  however,  is  an  error,  for 
there  is  no  equalising  of  such  different  planes.  But 
there  is  this  advantage :  that,  whereas  with  a  spherically 
corrected  lens,  when  employed  with  a  large  aperture, 
one  plane  of  the  face — the  eye,  for  example — is  rendered 
microscopically  sharp,  the  other  planes — such  as  the  ears 
and  nose — are  indistinctly  delineated  from  being  out  of 
focus,  in  a  '  diffusion '  lens  these  various  planes  appear  to 
possess  a  greater  equality  of  definition,  owing  to  the 
destruction  of  that  excessive  sharpness  of  one  plane  by 
which  the  others,  by  comparison,  were  degraded. 

Advantage  of  Diffusion. — We  are  not  now  speaking  of 
that  depth  of  focus  (which,  we  have  shown,  cannot  exist 
from  the  strictly  optical  point  of  view),  or  depth  of 
definition  which  arises  from  reducing  the  working  aper- 
ture of  a  lens,  but  of  that  quality  of  non-optical  definition 
arising  from  spherical  aberration  in  the  objective.  Now, 


APLANATJSM.  129 

while  we  like  a  lens  that  shall  '  cut  sharp  as  a  razor/  we 
also  like  the  power,  when  occasion  demands,  of  making 
a  picture  that  shall  not  be  quite  so  sharp. 

This  is  a  very  natural  want  felt  by  every  photographer 
who  does  not  consider  the  acme  of  perfection  to  lie  in  de- 
finition. Mr.  Fox  Talbot  found  the  need  of  such  a  power 
even  when  using  paper  negatives,  and  recommended  the 
separation  of  the  negative  from  the  sensitive  paper  by  the 
interposition  of  a  sheet  of  thin  paper  or  gelatine  as  a 
means  of  obtaining  this  requirement.  Others  have  sug- 
gested putting  the  sensitive  plate  a  little  out  of  focus  ; 
but  an  objection  to  this  is  found  in  the  fact  that  if  the 
face  of  the  sitter  be  out  of  focus  some  other  portion  will 
be  sharp,  and  Charybdis  is  no  better  than  Scylla.  If  a 
lens  have  a  moderately  large  aperture,  and  is  not  only 
properly  achromatised  but  aplanatic,  it  is  impossible  to 
escape  this  extra-sharp  definition  of  one  plane.  Every 
possessor  of  a  large  telescope  is  well  aware  that  if  it  be 
focussed  sharply  upon  an  object  situated  at  a  distance 
of  a  mile  an  object  only  half  a  mile  away  is  altogether 
out  of  focus ;  and  so  it  is  with  photographic  lenses 
within  a  more  limited  range.  In  order  to  remove  this 
property  some  means  must  be  utilised  by  which  the  lens 
can  be  rendered  non-aplanatic. 

The  term  '  aplanatic/  we  here  pause  to  say,  was  first 
employed  by  a  Scotch  savant,  Dr.  Blair,  who  in  1791 
made  use  of  it  to  signify  certain  points  of  superiority  in 
lenses  which  he  had  constructed.  Its  application  since 
that  time  has  been  narrowed  down  to  signify  freedom  from 
Spherical,  in  contradistinction  to  chromatic,  aberration. 

K 


130  DIFFUSION  OF  FOCUS. 

The  first  Diffusion  of  Focus  Lens, — The  first  account 
upon  record  of  any  lens  in  which  the  aplanatism  could 
be  modified  at  will,  so  as  to  secure  either  sharpness  or 
'diffusion/  was  given  in  April,  1864,  in  the  course  of  a 
paper  read  before  the  Photographic  Society  of  Scotland 
by  the  author.  When  exhibiting  a  lens  which  he,  as  an 
amateur,  had  constructed  for  his  own  use,  he  directed 
special  attention  to  the  fact  that  by  a  slight  re-arrange- 
ment of  the  lenses,  operated  by  a  projecting  button 
working  in  a  slot  in  the  mount,  the  fine,  crisp  definition 
given  by  the  lens  in  its  original  state  was  eliminated, 
and  that  in  the  altered  condition  it  gave  a  picture 
generally  sharp  all  over  the  plate,  but  particularly  sharp 
nowhere.  '  The  lens,'  he  said,  *  suddenly  becomes  pos- 
sessed of  a  new  property,  which  is  the  much-disputed 
one  of  depth  of  focus,  or,  more  strictly,  depth  of  definition, 
covering  a  large  flat  field  without  any  stop  whatever.' 
This,  it  is  believed,  is  the  first  exhibition  of  any  lens  for 
which  such  a  property  was  claimed,  and  special  attention 
was  at  the  time  directed  to  the  advisability  of  securing 
a  lowered  degree  of  sharpness  in  this  mode  rather  than 
by  the  common  method  of  putting  the  subject  a  little 
out  of  focus.  It  is  fortunate  that  lenses  both  by  home 
and  foreign  makers  are  now  easily  procurable  in  which 
by  a  separation  of  the  back  lenses  the  focus  may  be 
blunted  in  any  desired  degree. 

Dallmeyer's  Diffusion  of  Focus  Objective.  —  We  have 
already,  when  describing  portrait  lenses  of  large  an- 
gular aperture,  referred  (at  page  78)  to  the  back  lens 
introduced  by  J.  H.  Dallmeyer,  with  the  special  object 


OPENING  THE  FIXED  DIAPHRAGM.  131 

of  introducing  any  desired  amount  of  spherical  aberra- 
tion by  the  separation  of  its  components.  The  posterior 
of  these  is  set  in  the  main  cell  in  such  a  manner  as  to 
be  separated  from  its  fellow  by  turning  a  graduated  ring. 

Optical  Perfection  not  necessarily  Desirable. — When,  at 
a  series  of  discussions  on  lenses,  at  the  London  Photo- 
graphic Club,  the  author  took  occasion  to  attribute  a 
certain  degree  of  blame  to  the  manufacturers  of  lenses 
— especially  those  of  the  *  rapid  '  and  '  portable '  class  of 
compounds — for  curtailing  their  usefulness  by  limiting 
the  aperture  in  the  fixed  stop  to  that  point  at  which 
optical  crispness  terminated,  the  representative  of  a  large 
manufacturing  firm  who  was  present  good-humouredly 
hurled  a  jocular  anathema  at  the  individual  in  question, 
whose  first  act,  he  said,  upon  obtaining  one  of  their 
lenses  was  invariably  to  put  it  in  the  turning  lathe  and 
open  out  the  fixed  stop  to  the  diameter  of  the  lenses. 
This  is  precisely  the  course  we  are  now  about  briefly  to 
advocate,  and  its  reasonableness  will  stand  or  fall  by  the 
soundness  of  the  reasons  adduced. 

Advantage  of  Opening  the  fixed  Diaphragm. — When  a 
lens  of  the  description  specified  gives,  with  its  fixed 
diaphragm,  black  definition  —  by  which  we  mean  the 
rendering  of  a  piece  of  printed  matter  in  an  unmistakably 
sharp,  black  manner  without  greyness  or  fuzziness — it 
may  be  considered  as  being  optically  perfect ;  but  as 
every  lens  will  do  this  when  it  is  stopped  down  to  a 
sufficient  degree,  the  question  for  consideration  is — What 
price  do  we  pay  for  this,  or  what  do  we  suffer  in  the 
way  of  cutting  off  the  illumination  ?  The  larger  the 


132  OPENING  THE  FIXED  DIAPHRAGM. 

aperture  of  the  lens  that  does  this  the  better  is  such 
lens  ;  and  in  making  a  selection  of  a  'rapid'  objective 
this  is  one  of  the  points  to  which  we  always  pay  special 
attention,  for  some  will  not  define  '  black '  unless  the 
fixed  stop  be  very  small.  Let  us  suppose  that  we  have 
got  an  objective  the  diameter  of  the  lenses  of  which  is 
two  inches,  the  fixed  stop  between  the  two  being  one 
and  a  quarter  inch.  If  with  such  a  working  aperture  it 
gave  black  definition,  we  would,  without  hesitation,  have 
this  fixed  stop  opened  up  to  such  an  extent  as  upon  trial 
would  merge  the  black  definition  of  the  lines  into  grey, 
occasioned  by  the  overlapping  rays  caused  by  the  intro- 
duction of  spherical  aberration.  It  might  be  necessary, 
in  order  to  have  this  accomplished,  that  the  fixed  aper- 
ture be  increased  to  such  an  extent  as  almost  to  show 
light  round  the  margin  of  the  movable  diaphragms,  and 
two  such  lenses  in  our  possession  have  been  opened  out 
to  that  extent.  The  advantages  secured  are — first,  the 
ability  to  take  a  photograph  with  a  far  briefer  exposure 
than  was  previously  possible  ;  and,  secondly,  the  ability 
to  take  a  portrait  in  which,  while  the  sharpness  is  still 
of  excellent  degree,  it  is  chastened  or  softened  by  the 
modicum  of  aberration  so  introduced. 

Now  the  gain  thus  secured  has  been  obtained  with- 
out any  loss  whatever ;  for,  if  the  razor-edge  definition 
of  the  objective  in  its  original  state  be  required  at  any 
time,  it  can  be  immediately  secured  by  the  insertion  of 
a  diaphragm,  by  which,  so  far  as  light  and  crispness  of 
definition  are  concerned,  the  lens  is  returned  to  its  first 
state.  We  are  informed  that  opticians  would  with  plea- 


FUZZ  Y  PICTURES.  133 

sure  send  out  their  lenses  with  the  fixed  stop  enlarged 
in  the  way  and  to  the  extent  here  suggested  were  it  not 
there  are  many  inexperienced  photographers  who  could 
not  use  aright  such  a  power  were  it  conferred  upon 
them,  and  who,  misunderstanding  the  reason  for  the 
increased  aperture,  would  be  apt  to  decry  the  lens  as 
being  deficient  in  definition.  While  we  sympathise 
with  the  opticians  in  the  force  of  this  objection,  we 
recommend  the  propriety  of  the  course  suggested  to 
those  who,  being  already  in  possession  of  objectives  of 
the  class  to  which  we  now  refer,  are  at  liberty  to  alter 
them  in  their  brass  work  as  they  see  proper.  To  tamper 
with  the  glasses  themselves  would  be  highly  irrational, 
the  ability  to  do  so  being  assumed. 

Mechanical  means  for  producing  Fuzzy  Pictures. — Some 
of  the  mechanical  means  employed  in  the  production 
of  portraits  in  which  extreme  sharpness  has  no  place 
are  rather  amusing.  Among  these  we  may  refer  to  a 
system  not  long  ago  patented  by  one  of  the  most 
eminent  photographers  of  New  York  City,  which  con- 
sists in  placing  between  the  camera  and  the  sitter  a 
gridiron  arrangement  containing  several  gas  jets,  by 
which  ascending  currents  of  air  of  varying  densities 
from  the  flames  disturb  the  sharpness  of  the  definition 
and  produce  an  alleged  greater  harmony.  The  pictorial 
results  are  designated  '  vibrotypes.'  A  similar  effect  is 
obtained  by  having  a  trembling  camera-stand,  or  by 
attaching  a  string  from  the  camera  to  the  floor  and 
causing  it  to  vibrate  during  exposure. 

Claudet's    System. — The    method    employed    by   M. 


134  DIFFUSION. 

Claudet  was  much  more  philosophical.  It  consisted 
in  moving  the  lens  in  and  out  of  the  camera,  within 
certain  limits,  during  the  seance^  so  that  whereas  at  the 
commencement  of  the  exposure  the  nose  may  have  been 
sharply  in  focus  and  the  eyes  or  ears  cut  of  focus,  or 
vice  versa,  at  the  conclusion  these  conditions  were 
changed,  the  nose  being  then  out  and  the  ears  in  focus. 
The  focus  was  thus  distributed  over  the  entire  plane  of 
the  face.  M.  Claudet  made  a  specialty  of  very  large 
portraits,  which  necessitated  the  employment  of  portrait 
lenses  of  large  dimensions  ;  and  there  is  no  doubt  that 
by  the  means  just  indicated  he  secured  equalised  defini- 
tion over  various  planes. 

Into  the  art  aspect  of  diffusion  of  focus  we  have 
avoided  entering,  our  attention  having  been  confined 
to  considering  the  question  from  the  optical  point  of 
view. 

Diffusion  by  Single  Achromatic  Lenses.— The  value  of 
a  single  achromatic  lens  of  plano-convex  or  meniscus 
form  in  producing  'diffused'  portraits  is  well  known. 
It  must  be  worked  with  a  stop  much  larger  than  would 
be  employed  in  landscape  work.  Portraits  of  large 
dimensions  and  great  technical  excellence  have  often 
been  obtained  by  such  agency. 


CHAPTER    XXIII. 

TESTING   LENSES. 

Preparation  of  Camera. — In  testing  a  lens  it  is  im- 
portant that  the  ground  glass  of  the  camera  be  so 
smooth  or  of  such  a  fine  grain  as  to  permit  of  the 
use  of  a  magnifying  glass  without  the  image  suffering 
from  granularity.  The  mere  masking  of  this  granularity 
by  waxing  or  oiling  the  surface  of  the  focussing-screen 
will  not  suffice  ;  the  grain  must  be  fine  in  itself. 

It  is  equally  important  that  the  surface  of  the  ground 
glass  be  at  precisely  the  same  distance  from  the  lens 
as  that  of  the  sensitive  plate.  This  cannot  be  ascer- 
tained with  the  requisite  accuracy  by  the  usual  method 
of  pushing  a  foot-rule  through  the  aperture  in  the  front 
of  the  camera,  observing  how  far  it  goes,  and  then  trying 
in  the  same  way  a  plate  in  the  dark  slide.  A  more 
accurate  method  consists  in  laying  a  straight  rule  across 
the  focussing-glass  frame,  and  inserting  between  the 
edge  of  the  rule  and  the  surface  of  the  glass  a  slip 
of  card  cut  in  the  form  of  a  wedge,  and  observing  the 
distance  it  can  be  inserted,  making  a  pencil  mark  at 
the  place  where  it  touches  the  rule.  Next  insert  a 
plain  glass  in  the  camera  dark  slide,  and  do  likewise. 


;36  TESTING  LENSES. 

If  the  point  of  contact  of  the  wedge  be  the  same  in 
both  cases  then  both  planes  are  coincident. 

In  this  way  a  difference  of  a  hundredth  part  of  an 
inch  between  the  plane  of  the  ground  glass  and  of  the 
sensitive  plate  may  readily  be  detected.  If  the  wooden 
adapters  in  the  dark  slide  be  thin  and  the  spring  in 
the  back  be  strong,  there  is  a  danger  of  the  sensitive 
plate  being  forced  nearer  to  the  lens  than  it  ought  to 
be,  and  the  focussing  thus  disturbed.  More  than  one 
optician  of  eminence  has  had  lenses  of  large  aperture 
and  unmistakable  excellence  returned  for  alteration 
owing  to  an  imaginary  fault  caused  by  the  strength 
of  the  spring. 

The  Points  to  be  Tested. — These  are  various  and  will  be 
treated  individually.  They  comprise  —  covering  power 
or  area  of  illumination  ;  achromatism,  actinism,  or  co- 
incidence of  visual  and  chemical  focus  ;  astigmatism  ; 
flatness  of  field  ;  surface  finish  ;  striae  and  air  bubbles  ; 
purity  of  glass  ;  definition  ;  flare  ;  focus  ;  rectilinearity  ; 
aplanatism  ;  and  spherical  aberration. 

Although  these  topics  are  treated  in  the  other 
chapters,  yet  it  may  be  well  here  to  devote  a  few 
words  to  each. 

Covering  Power  or  Area  of  Illumination. — The  area  of 
illumination  is  circular,  and  its  diameter  determines  the 
size  of  plate  that  can  be  got  out  of  such  a  circle.  We 
are  not  at  present  referring  to  the  quality  of  the  image 
that  may  be  produced  from  centre  to  margin  of  such 
area,  which  may  be  good  or  bad,  but  to  the  mere 
illumination  to  the  corners.  The  diameter  of  this  circle 


A  CHROMA  TISM.  137 

equals  the  diagonal  of  any  plate  (that  is,  measured  from 
coiner  to  corner),  which  will  be  lighted  to  the  corners. 
Take  the  case  of  a  whole-plate,  i.e.,  one  of  8J  by  6£ 
inches,  the  diagonal  of  this  is  loj  inches,  and  no  lens 
giving  a  less  area  of  illumination  than  this  latter  figure 
will  cover  the  plate.  But  if  a  panorama  were  wanted, 
then  by  employing  a  plate  of  only  3^  inches  in  height 
and  9!  in  length,  a  greater  angle,  measured  on  the  base, 
could  be  included.  Let  the  possessor  of  any  lens  ascer- 
tain the  diameter  of  its  area  of  illumination  and  draw 
this  on  a  sheet  of  paper,  he  can  then  by  placing  any 
plate  upon  this  circle  see  at  a  glance  whether  or  not  the 
lens  will  cover  it. 

Achromatism  or  Actinism. —  The  focussing  screen 
having  been  adjusted  accurately,  it  is  next  desirable 
to  ascertain  if  the  lens  has  a  chemical  focus,  or,  in  other 
words,  whether  the  actinic  and  visual  foci  be  so  carefully 
adjusted  that  both  shall  fall  on  the  same  plane.  Place 
seven  or  eight  printed  cards  in  a  row  on  edge  on 
a  slab  of  wood,  the  distance  between  each  being  six 
inches.  In  addition  to  the  printed  matter,  each  card 
should  be  boldly  inscribed  with  a  figure  in  black  ink. 
Having  placed  the  slab  on  a  table  at  a  distance  of  ten 
feet,  arrange  so  that  the  cards  shall  be  all  focussed  as 
near  the  centre  of  the  ground  glass  as  possible,  all  of 
them  being  shown.  By  the  aid  of  a  magnifier  focus 
sharply,  without  using  a  stop,  the  centre  figure  of  the 
row,  which,  if  seven  cards  are  employed,  will  be  marked 
'4.'  Now  insert  a  sensitive  plate  and  take  a  picture; 
and,  if  on  the  subsequent  negative  the  fourth  card  be 
sharper  than  the  others,  it  proves  the  coincidence  of  the 


138  OVER  OR  UNDER  CORRECTION. 

two  foci.  Should,  however,  a  card  further  away  than 
that  focussed  upon  be  found  to  be  the  sharpest  in  the 
negative,  it  indicates  that  the  lens  is  over-corrected  for 
colour,  or,  as  expressed  by  some,  it  has  a  back  focus. 

Visual  Test  for  Over  or  Under  Correction. — At  this 
juncture  it  may  be  desirable  that  we  give  an  easy 
method  for  ascertaining  whether  a  lens  has  its  blue 
and  yellow  rays  brought  to  the  same  focus,  or  is 
'under -corrected'  for  colour,  which  is  the  necessary 
condition  in  a  photographic  objective.  Bring  the  lens 
to  be  examined  into  a  slightly  darkened  room  in  which 
there  is  a  gas-light  burning,  and,  retreating  several  feet 
from  it,  hold  up  the  lens  so  as  to  form  an  image  of  this 
light  in  the  eye  of  the  observer.  The  image  must, 
however,  be  examined  through  an  eyepiece  of  any 
good  construction  ;  we  prefer  the  *  Ramsden '  for  this 
purpose.  At  the  point  where  the  image  is  sharpest 
there  is  but  little  colour ;  but,  by  bringing  the  portrait 
lens  a  little  nearer,  the  flame,  if  the  lens  be  properly 
corrected,  is  seen  to  be  surrounded  with  a  claret  fringe, 
while  on  removing  it  to  a  greater  distance  than  distinct 
definition,  the  light  is  fringed  with  green,  proving  that 
the  blue  and  yellow  rays  are  combined,  and,  as  a  con- 
sequence, that  the  chemical  and  visual  foci  coincide. 

When  a  lens  is  not  properly  corrected  for  colour, 
over -correction  is  the  direction  in  which  the  error 
usually  lies,  especially  in  foreign  Petzval  portrait  com- 
binations, and  in  almost  every  instance  which  has  been 
brought  under  our  observation,  the  front  lens  has  been 
the  defaulter. 


tZSTING  FOR  ASTIGMATISM.  139 

Astigmatism. — Astigmatism  is  a  serious  fault  for  a 
lens  to  possess  in  any  marked  degree.  It  is  closely 
allied  with  flatness  of  field — that  is  to  say,  it  is  usually 
produced  in  the  endeavour  to  make  a  lens  which  will 
cover  a  flat  field  with  a  large  aperture.  A  lens  of  this 
class  will  work  quite  sharply  in  the  centre,  but  in  pro- 
portion as  an  object  (such  as  the  head  of  a  sitter)  is 
made  to  occupy  a  position  tolerably  far  from  the  centre 
of  the  plate  so  does  the  sharpness  diminish,  and  no 
amount  of  racking  the  lens  in  or  out  will  give  definition 
equal  to  that  in  the  centre.  To  test  a  lens  for  astig- 
matism, erect  a  black  cross  against  a  white  background. 
What  we  find  most  convenient  for  the  purpose  are  the 
astragals  of  an  ordinary  window.  At  any  rate  there 
must  be  a  vertical  line  crossed  by  a  horizontal  one. 
Now  focus  these  sharply  on  the  centre  of  the  ground 
glass,  and  it  will  be  found  that  both  lines,  the  vertical 
and  horizontal,  are  well  delineated  and  equally  distinct. 
Next  rotate  the  camera  slightly  so  as  to  bring  the 
crossed  lines  to  either  the  side  or  the  top  or  bottom 
of  the  focussing-screen,  and  again  examine  the  image 
very  carefully,  when  the  want  of  sharpness  will  be  most 
apparent.  Rack  the  lens  in  and  out,  and  a  point  will 
be  found  at  which  the  horizontal  bars  will  be  sharp, 
while  the  vertical  ones  are  so  far  out  of  focus  as  to  be 
almost  invisible,  or,  at  any  rate,  to  have  their  sharp- 
ness greatly  impaired.  Now  manipulate  the  rack  once 
more,  and  the  vertical  lines  will  become  sharp,  leaving, 
this  time,  the  horizontal  ones  as  a  confused  mass 
of  indistinctness. 


i4o  FLATNESS  OF  FIELD. 

In  a  similar  manner,  provide  a  sheet  of  brown  paper 
with  a  round  hole  in  it,  and  fix  it  on  the  window. 
Direct  the  camera  to  it  as  before,  and  observe  that 
when  the  image  is  thrown  on  the  focussing-screen  it 
is  quite  round,  no  matter  whether  the  lens  be  racked 
within  or  without  the  point  of  true  focus.  Now  rotate 
the  camera  so  as  to  bring  the  image  to  the  margin,  as 
in  the  previous  experiment,  and,  behold  !  it  is  no  longer 
round  as  before ;  for,  when  the  lens  is  racked  in  or  out, 
it  becomes  alternately  elongated  vertically  or  horizontally, 
according  as  the  lens  is  nearer  to  or  further  from  the 
ground  glass  than  the  best  mean  focus.  When  the 
lens  is  made  to  approach  the  focussing-screen,  the 
luminous  spot  is  elongated  vertically ;  but  when,  on 
the  contrary,  the  focus  is  lengthened,  the  spot  expands 
horizontally. 

It  is  only  in  lenses  corrected  for  great  flatness  of 
field  that  astigmatism  is  usually  to  be  found  in  a 
strongly  marked  degree,  although  it  is  present  to  a 
slight  extent  in  almost  every  lens.  A  portrait  lens, 
however,  having  a  round  field,  is  more  likely  to  possess 
freedom  from  it  than  any  other.  The  skilful  optician 
constructs  his  objectives  so  as  to  have  as  little  astig- 
matism with  as  much  flatness  of  field  as  possible. 

Flatness  of  Field, — The  best  lens  is  that  one  which, 
giving  brilliant  definition  at  the  centre  with  a  large 
aperture,  shall  with  the  same  aperture  maintain  that 
brilliance  and  definition  farthest  away  from  the  centre 
of  the  plate.  Place  the  camera  opposite  any  row  of 
well  marked  objects  not  within  several  yards — a  row  of 


SURFACE  FINISH-STRIDE.  141 

brick  houses  will  answer — and  focus  with  the  greatest 
care  on  the  centre  of  the  ground  glass.  Note  the  extent 
of  crisp  definition,  and  how  near  it  approaches  the  side 
of  the  picture.  It  will  also  do  to  focus  on  one  well- 
marked  object  in  the  centre  of  the  field  and  rotate  the 
camera,  observing  how  much  it  falls  away  when  brought 
to  the  edge  of  the  ground  glass,  and  how  much  racking 
in  is  required  to  make  it  sharp  there. 

Surface  Finish. — The  quality  of  this  property  is  ascer- 
tained by  holding  the  lens  against  the  light,  gas  being 
preferred  to  daylight,  and  examining  its  surfaces  with  a 
watchmaker's  eyeglass  or  similar  powerful  glass.  In 
this  way  lenses  which  have  been  imperfectly  polished, 
or  not  properly  smoothed  at  the  stage  prior  to  receiving 
the  final  polish,  will  be  discovered  to  have  a  slightly 
granular  surface.  We  have  known  lenses  of  this  sort 
perform  well,  but  it  is  none  the  less  a  defect  which 
ought  not  to  exist. 

Striae  and  Air  Bubbles. — Striae  in  the  glass  is  discover- 
able by  taking  the  lens  into  a  room  from  which  daylight 
is  excluded,  and,  turning  the  gas  rather  low,  examining 
the  image  by  the  gas.  Step  a  few  feet  back  from  the 
light,  and  holding  up  the  lens  so  that  the  whole  surface 
appears  one  mass  of  light,  move  it  slightly  from  side  to 
side,  and  turn  it  partially  around.  In  this  way  a  very 
slight  inequality  in  the  density  of  the  glass,  or  a  want 
of  homogeneity  in  its  composition  will  be  discovered,  if 
such  be  present.  Air  bubbles,  if  only  of  small  size,  are 
not  of  the  same  consequence  as  striae,  for  they  do  not 
affect  the  definition.  As  no  light  is  radiated  from  them, 


142  DEFINITION. 

they  act  only  as  would  so  many  specks  of  opaque  matter 
of  the  same  dimensions.  The  testing  of  glass  previous 
to  being  ground  into  a  lens  is  spoken  of  in  another 
chapter. 

Purity  of  the  Glass. — The  quickest  acting  lenses, 
c&teris  paribus,  are  those  the  glasses  of  which  are  colour- 
less. In  some  of  the  oldest  combinations  the  crown 
glass  element  was  of  a  pronounced  green  colour,  which 
interfered  much  with  their  rapidity.  To  ascertain  the 
purity  of  the  glass,  as  regards  colour,  place  the  lens 
upon  a  sheet  of  white  paper  and  note  the  degradation 
of  colour,  if  any,  that  takes  place  when  looking  down 
upon  it.  If  the  discoloration  be  of  a  brown  character, 
the  lens  will  prove  slower  in  action  than  if  it  be  quite 
colourless.  Some  who  have  much  work  in  copying 
paintings  or  coloured  prints  assert  that  they  get  a  truer 
rendering  of  the  value  of  colours  when  using  a  lens  of 
dingy  colour  than  with  one  formed  of  purer  glass.  In 
such  a  case  the  lens  itself  enacts  the  part  of  the  colour- 
screen  of  pale  yellow  glass  often  employed  to  attain  a 
similar  end. 

Definition. — One  of  the  best  test  objects  for  definition 
in  a  lens  is  an  enamelled  watch  dial  with  the  seconds 
circle.  Let  this  be  placed  at  a  distance  of  a  few  yards 
and  well  lighted,  either  by  lamp  or  daylight.  On  focus- 
sing sharply,  ascertain  that  the  division  between  the 
black  strokes  forming  chapters  two,  three,  four,  and 
twelve,  and  also  the  seconds,  are  all  well  made  out. 

Flare  Spot, — Unlike  the  other  tests,  this  one  should 
be  applied  after  a  diaphragm  has  been  inserted  in  the 


FOCUS— RECTILINEARITY.  143 

lens.  Let  the  camera  be  directed  against  a  rather  dark 
object,  such  as  a  tree  in  foliage,  with  a  bright  sky  over- 
head ;  an  ordinary  window  will  answer,  provided  the 
lower  portion  be  obscured  by  a  sheet  of  dark  paper. 
If  there  be  a  flare  spot,  it  will  be  seen  in  the  centre  of 
the  ground  glass.  This  spot,  as  we  have  explained  in  a 
former  chapter,  is  an  image  of  the  diaphragm,  and  single 
lenses  as  well  as  combinations  are  liable  to  it.  For- 
tunately it  is  easily  remedied.  This  test  can  also  be 
made  in  a  room  lighted  by  gas  or  lamp. 

Focus, — The  various  methods  by  which  the  focus  of 
a  lens  is  known  are  so  fully  described  in  Chapter  VI. 
that  we  refer  the  reader  to  it,  especially  as  some  of  the 
systems  may  from  facility  of  application  or  otherwise 
be  preferred  by  some  rather  than  others,  and  to  cite 
them  here  would  be  but  unnecessary  repetition. 

Rectilinearity. —  Place  the  camera  quite  level,  and 
direct  it  towards  any  perfectly  straight  object,  such  as 
the  wall  of  a  house,  the  side  of  a  straight  window,  or, 
in  short,  to  anything  that  is  quite  straight,  and,  having 
focussed  it  in  the  centre  of  the  screen,  rotate  the  camera 
until  the  image  is  brought  close  to  the  margin.  Note 
whether  the  image  is  now  curved  or  if  it  preserves  its 
straightness.  In  the  latter  case  the  lens  is  quite 
rectilinear. 

How  to  Cure  Over-Correction. — There  are  three  methods 
by  which  the  evils  arising  from  over-correction  may  be 
cured.  The  first  is  that  which  will  prove  the  most 
effectual  and  give  the  least  trouble  in  future.  It  con- 
sists in  removing  the  lens  from  its  cell,  separating  its 


1 44  0  VER-  CORRECTION. 

components  by  immersion  in  water  sufficiently  warm  to 
soften  the  Canada  balsam  by  which  the  lens  is  cemented, 
and  then  regrinding  the  contact  surfaces  in  tools  of 
greater  radius  of  curvature.  Only  few  photographers 
are  able  to  execute  work  of  this  sort  for  themselves ; 
for,  although  many  are  quite  facile  in  effecting  any 
manipulation  or  original  investigation  in  chemistry, 
others  being  equally  expert  in  mechanical  and  artistic 
departments,  the  number  of  those  who  have  entered 
the  field  of  optics  by  way  of  experiment  or  amuse- 
ment is  very  limited.  A  'jobbing'  optician  will  be 
more  likely  to  undertake  the  regrinding  of  a  lens  than 
the  manufacturing  optician,  who  could  scarcely  be  ex- 
pected to  go  out  of  his  way  to  execute  a  trivial  order 
of  this  nature. 

A  method  which  was  much  employed  when  over- 
corrected  lenses  were  more  commonly  to  be  met  with 
than  is  now  the  case  consisted  in  having  a  graduated 
scale  engraved  on  the  sliding  tube,  so  that  when  a 
visual  image  was  focussed  sharply  on  the  ground  glass, 
the  lens  had  then  to  be  racked  out  a  certain  number 
of  degrees  in  order  to  ensure  the  image  being  sharp 
in  the  negative.  This  distance  is  a  constant  one  only 
for  an  object  situated  a  definite  space  from  the  camera 
or  in  the  major  conjugate  focus  of  the  lens,  and  varies 
with  every  distance  of  such  object.  Were  this  not  the 
case  it  would  be  easy  to  sink  the  ground  glass  deeper 
in  its  frame,  by  which  the  same  end  would  be  achieved. 
If  a  lens  of  this  class  must  be  employed — and  it  is  a 
well- recognised  fact  that  some  will  produce  photographs 


APLANATISM.  145 

as  sharp  and  fine  in  every  respect  as  those  in  which 
the  actinic  and  visual  foci  coincide — the  best  way  by 
far  to  utilise  them  with  a  minimum  of  trouble  and  with 
freedom  from  all  uncertainty  is  to  adopt  the  system 
described  in  Chapter  VI.,  which  consists  in  inserting,  in 
the  manner  of  a  Waterhouse  diaphragm,  a  very  weak 
lens,  the  power  of  which  shall  be  such  as,  when  inserted 
in  its  place,  to  lengthen  the  focus  of  the  objective  to 
an  extent  equalling  the  difference  between  the  visual 
and  chemical  foci.  If,  then,  the  object  be  focussed 
\vhen  this  auxiliary  lens  is  inserted,  and  the  lens  be 
then  withdrawn  when  the  exposure  is  about  to  be  made, 
the  image  will  be  quite  sharp.  It  may,  perhaps,  be 
scarcely  necessary  to  observe  that  in  all  cases  when 
purchasing  a  lens  we  recommend  that  one  having  a 
'  chemical  focus  '  should  be  avoided. 

Aplanatism  and  Spherical  Aberration. — To  test  a  portrait 
combination  for  spherical  aberration,  the  Shadbolt 
method  is  as  good  as,  and  more  convenient  than,  any 
other.  Cut  a  disc  of  thick  brown  paper  of  the  same 
diameter  as  the  front  lens  of  the  combination,  and  from 
the  centre  of  this  cut  out  a  smaller  disc  seven-tenths  of 
the  entire  diameter.  There  is  thus  a  disc  and  a  ring, 
the  areas  of  which  differ  only  a  trifle  from  one  another. 
Now,  first  insert  the  ring  of  brown  paper,  which  will  act 
as  a  diaphragm  ;  and,  having  carefully  focussed  on  a 
printed  bill,  take  an  impression,  which  should  be  clear 
and  sharp.  Next  remove  the  ring  of  paper,  and  with 
a  little  gum  or  paste  attach  the  paper  disc  to  the  centre 
of  the  lens,  and  without  altering  the  focus  take  another 


146  SPHERICAL  ABERRATION. 

picture  of  the  bill.  If  the  spherical  aberration  be  at  all 
well  corrected,  the  second  picture  should  be  nearly  as 
sharp  as  the  first ;  but  if  the  correction  be  insufficient 
the  latter  picture  will  be  more  or  less  indistinct. 

Again,  focus  some  well-marked  test  object  —  the 
small  bare  branches  of  a  tree  against  the  sky  will 
answer — without  any  stop,  using  an  eyeglass  to  ensure 
accuracy,  and  mark  the  position  on  the  camera.  Next 
insert  a  rather  small  diaphragm,  and  rack  the  camera 
in  and  out  till  the  greatest  point  of  sharpness  is  as- 
certained. Mark  the  camera  again,  and  then  ascertain 
if  the  two  marks  quite  coincide.  If  they  do,  then  is  the 
combination  aplanatic  or  spherically  corrected. 


CHAPTER  xxiv. 

THE  SHAPE  OF  THE  APERTURE  IN  THE   DIAPHRAGM. 

Fallacies  respecting  Shapes  of  Apertures. — A  popular 
fallacy  existed  at  one  time  in  a  greater  degree  of 
strength  than  at  present  to  the  effect  that  the  shape 
of  the  aperture  in  the  diaphragm  should  bear  a  certain 
relation  to  the  general  form  of  the  principal  subject  in 
the  photograph.  For  example  :  a  vertical  slit  instead 
of  a  round  hole  was  believed  to  be  the  correct  form 
when  the  subject  was  tall,  such  as  a  church  spire  or 
other  similar  vertically  elongated  subject. 

In  the  case  of  portraiture  an  aperture  somewhat  like 
a  keyhole  has  been  proposed  as  that  best  adapted  for 
this  class  of  subject,  while  for  landscapes  some  virtue  is 
still  by  some  imagined  to  be  imparted  to  the  illumina- 
tion of  the  image  if  the  aperture  be  wide  at  the  base 
and  tapered  off  to  a  fine  point  at  the  top,  the  imagined 
advantage  consisting  in  a  greater  volume  of  light  being 
permitted  to  reach  the  foreground  than  that  by  which 
the  sky  is  produced. 

Circular  Apertures  Best. — The  best  shape  of  aperture 
is  circular,  and  the  next  best  such  a  degree  of  departure 
from  the  circular  form  as  shall  most  nearly  confine  the 

1.7. 


i4&      FANCIFUL  APERTURES  OBJECTIONABLE. 

transmitted  rays  to  a  condensed  bundle.  This  embraces 
a  circle  (the  iris  diaphragm)  formed  of  several  blades, 
by  the  motion  of  which,  regulated  by  a  volute,  the  aper- 
ture may  be  expanded  or  contracted  to  a  large  extent, 
while  a  sufficient  approximation  to  the  circular  form  is 
still  maintained.  Next  to  this  comes  a  square,  which  by 
the  motion  of  two  plates  in  opposite  directions  —  as  first 
described  by  the  late  M.  Noton  —  is  also  applicable  to 
an  easy  formation  of  an  expanding  and  contracting 
aperture.  The  worst  forms  of  all  are  those  whimsical 
ones  shaped  sometimes  like  a  bottle,  sometimes  like  a 
pyramid  erected  on  a  circle,  and,  worse  than  all,  like 
a  slot. 

It  is  not  difficult  to  give  a  reason  for  such  con- 
demnation. Take  the  case  of  a  sky  and  foreground  as 
an  example.  For  such  a  subject  an  aperture  of  an  ex- 
ceedingly tall  pyramidal  shape  has  been  recommended 
as  possessing  advantages  over  others.  This  recom- 
mendation has  been  made  by  individuals  who  are  not 
considered  mere  '  nobodies '  in  photography,  otherwise 
it  might  be  allowed  to  pass  without  reference ;  but  it  is 
worthy  of  notice  that  the  recommendation  has  not  been 
backed  up  by  a  single  argument  of  a  scientific  nature. 
They  imagine  it  ought  to  be  so,  and  think  that  it  really 
is  so ;  and  there  the  demonstration  ends.  Let  us  see 
in  what  manner  this  wedge -shape  slot  or  aperture 
affects  the  foreground  as  contrasted  with  the  sky  of  the 
landscape. 

In  a  previous  chapter  it  has  been  shown  that,  in  a 
landscape  lens,  the  margin  of  the  picture  must  be  formed 


CIRCULAR  APERTURES  BEST.  149 

by  the  margin  of  the  lens,  the  same  conditions  pre- 
vailing with  the  centre  of  the  photograph.  Any  depar- 
ture from  this  is  attended  by  disadvantages,  such  as 
spherical  aberration.  In  order  that  any  photographer 
may  satisfy  himself  that  the  shape  of  the  diaphragm 
goes  for  nought  in  reducing  the  intensity  of  light  upon 
the  sky,  it  merely  suffices  that  after  placing  the  camera 
in  position  in  front  of  a  landscape  he  then  removes  the 
ground  glass.  Now,  having  placed  his  eye  where  the 
sky  on  the  ground  glass  was,  let  him  direct  his  vision 
towards  the  stop.  This  will  demonstrate  to  the  observer 
that  he  can  see  the  whole  of  the  aperture  in  the  dia- 
phragm. Let,  now,  the  same  thing  be  done  from  the 
position  occupied  by  the  sky,  and  precisely  the  same 
amount  of  aperture  in  the  diaphragm  is  seen,  showing 
that  whimsicality  in  shape  goes  for  nothing  in  regard 
to  illuminating  one  portion  of  the  picture  more  than 
another. 

This  applies  also  to  the  use  of  either  a  vertical  or 
horizontal  slit  instead  of  a  circular  hole.  If  a  set  01 
parallel  oblique  rays  fall  upon  the  lens  they  do  not  all 
proceed  in  the  same  direction  after  transmission  ;  but, 
according  to  the  principles  of  spherical  aberration,  the 
focus  of  a  pencil  transmitted  by  that  side  of  the  lens 
farthest  removed  from  the  object  whence  the  rays 
emanate  will  be  much  longer  than  those  transmitted 
by  the  nearer  margin  of  the  lens.  Hence  a  slit  aperture 
will  give  confusion  ;  but  if  a  round  aperture  be  sub- 
stituted, all  such  confusion  will  cease  to  exist. 


CHAPTER  XXV. 

EQUALISING  THE  ILLUMINATION   OF  SUBJECTS  — 
SKIES  AND  FOREGROUNDS. 

How  to  Obviate  the  Excessive  Light  from  the  Sky. — It 
is  not  only  possible  but  quite  easy  to  arrange  a  stop  so 
that  it  will  admit  a  much  larger  volume  of  light  to  the 
foreground  of  a  landscape  image  than  to  the  sky.  Not 
only  so,  but  if  one  side  of  a  subject  were  in  deep  shadow 
or  of  a  dark  colour  —  such  as  a  dense  mass  of  trees  on 
one  side  placed  in  contrast  with  a  sunny,  well-lighted 
object  on  the  other  —  it  is  comparatively  easy  so  to 
arrange  matters  as  that  one  side  will  receive  a  more 
intense  pencil  of  light  than  the  other. 

Much  ingenuity  has  of  late  been  displayed  in  the 
construction  of  shutters  which,  in  falling,  wUl  permit  of 
a  longer  exposure  being  given  to  the  foreground  than 
to  the  sky.  But  this  can  be  obtained  equally  well  by 
means  of  a  shutter  of  the  ordinary  class,  or  by  a  pro- 
longed exposure,  provided  the  diaphragm  be  placed 
oblique  to  the  axis  of  the  lens. 

The  Oblique  Diaphragm.  —  In  demonstration  of  the 
foregoing  we  refer  to  Fig.  48,  in  which  a  represents  a 
lens  of  any  form  ;  d  is  a  diaphragm  placed  at  a  slope 


OBLIQUE  DIAPHRAGM.  151 

instead  of  the  right  angle  at  which  it  is  usually  fixed. 
In  this  position  it  is  directed  downwards  towards  the 
foreground  or  less-lighted  portion  of  the  subject,  the 
consequence  of  this  being  that  the  large  volume  of  light 
bounded  by  the  lines  r,  r't  and  which  comes  from  the 


FIG.  48. 

foreground,  exceeds  by  many  degrees  that  coming  from 
the  sky  shown  at  s,  / ;  and  these  arrive  at  their  respec- 
tive foci/,/',  the  one  in  a  state  of  great  attenuation  in 
comparison  with  the  other. 

The  principle  of  the  oblique  stop  is  the  same  whether 
it  be  applied  to  a  single  landscape  lens,  as  in  the  figure, 
or  to  a  combination.  But  we  have  found  opticians  very 
reluctant  to  adapt  this  oblique  system  to  any  lens.  The 
usual  working  appliances,  we  were  told,  did  not  embrace 
the  easy  or  effective  cutting  of  a  slot  obliquely  in  the 
mount.  To  describe  the  several  mechanical  expedients 
we  found  it  advantageous  to  adopt  in  having  stops  so 
arranged  as  to  be  capable  of  standing  at  any  desired 
angle  would  be  rather  out  of  place  in  this  chapter,  espe.- 


152  EQUALISING  BY  OPAQUE  STOP. 

daily  as  the  mere  indication  of  the  remedy  for  under- 
exposed foregrounds  is  all  that  is  here  required. 

Equalising  by  an  Opaque  Stop. — A  system  which  we 
adopted  a  few  years  ago,  with  exceedingly  satisfactory 
results,  consists  in  placing  at  a  little  distance  in  front  of 
the  diaphragm  a  small  piece  of  blackened  brass  of  a 
V  shape,  base  upwards.  One  or  two  trials  will  suffice 
to  determine  its  best  position.  This  fulfils  the  following 
conditions  : — It  gives  a  proportionately  greater  illumina- 
tion to  the  foreground  than  to  the  sky,  and,  while  it 
diminishes  to  any  required  extent  the  intensity  of  the 
light  which  falls  upon  the  centre  of  the  plate,  it  gives  a 
great  increase  to  that  by  which  the  sides  are  illuminated. 
Added  to  these,  it  costs  nothing,  and  can  be  applied  by 
any  photographer  to  his  lens  without  any  disfigurement 
of,  or  tampering  with,  the  brass  work  ;  for  the  whole 
appliance  can  easily  be  made  and  fixed  in  a  couple  of 
minutes  by  means  of  a  pair  of  scissors,  a  bit  of  stiff 
black  paper,  and  a  little  mucilage.  When  making  our 
original  experiments  with  this  device  we  actually  suc- 
ceeded in  turning  the  tables  so  that  the  foreground  was 
far  better  illuminated  than  the  sky,  and  the  margins 
much  more  so  than  the  centre,  a  wide  angle  of  subject 
being  included. 

The  unequal  illumination  of  a  negative,  especially 
one  of  wide  angle,  is  due  to  two  causes.  The  centre 
receives  a  more  intense  impact  of  light  than  the  sides 
on  account  of  the  pencil  of  light  transmitted  to  it  being 
both  larger  and  having  a  shorter  distance  to  travel  from 
the  lens  to  the  sensitive  surface.  Not  so  with  the 


BOW'S  METHOD.  153 

margins  ;  for  the  stop  with  its  circular  aperture  being 
placed  obliquely  as  regards  the  margin  the  aperture  is 
not  then  circular,  but  oval — a  matter  easily  verified  by 
looking  through  a  stop,  first  directly,  and  then  when 
turned  in  an  oblique  direction.  This  renders  the  oblique 
light  less  to  begin  with ;  but  this  attenuated  light  has 
also  got  much  farther  to  travel  than  the  stronger  central 
bundle,  and  hence  the  marginal  weakness. 

The  Butterfly  Stop.— In  the  case  of  ordinary  angles 
of  view  this  difference  is  so  little  as  not  to  merit  much 
attention ;  but  this  is  not  so  in  the  case  of  highly  ob- 
lique incidences.  To  equalise  the  light  by  means  of  the 
stop  in  his  panoramic  camera  the  late  Thomas  Sutton 
devised  a  little  adjunct  of  great  ingenuity.  It  was  a 
stop  which,  no  matter  whether  held  at  right  angles  and 
looked  at  directly  or  at  a  very  oblique  angle,  always 
presented  a  perfectly  circular  aperture.  This  was 
effected  by  two  thin  little  wings  of  brass  screwed  upon 
the  stop  in  such  a  manner  as  to  effect  the  equalisation 
required  (see  Figure  28,  page  66). 

Bow's  Method  of  Equalising. — We  close  this  chapter 
by  alluding  to  one  other  method  suggested  (by  Robert 
H.  Bow,  C.E.)  for  causing  the  iens  itself  to  be  the 
equalising  medium.  It  consists  in  having  the  crown  or 
plate  glass  element  of  the  lens  of  a  delicate  green  colour, 
by  which  the  thick  centre  will  stop  more  actinic  rays 
than  the  thin  margin,  the  other  portions  of  the  lens 
acting  in  an  intermediate  degree, 


CHAPTER  XXVI. 

ADJUSTING   DISSIMILAR    LENSES. 

Matching  Stereoscopic  Lenses. — In  matching  a  pair  of 
lenses  for  stereoscopic  purposes,  serious  difficulties  have 
not  unfrequently  to  be  encountered.  This  difficulty 
scarcely,  if  ever,  prevails  when  the  lenses  are  of  the 
single  landscape  class,  but  is  most  apparent  in  the  case 
of  combinations.  Indeed,  the  difficulty  of  rinding  two 
portrait  objectives  so  identical  in  focus  as  to  produce 
pictures  which,  as  respects  dimensions,  will  be  facsimiles 
is  well  recognised.  Even  when  a  number  of  lenses  are 
made  out  of  the  same  pot  of  glass,  and  ground  to  the  same 
curves,  marked  differences  will  often  exist  in  their  foci. 

Such  being  the  case  with  lenses  of  a  similar  kind 
coming  from  one  optician,  the  difficulty  of  obtaining  two 
lenses  alike,  which  have  been  made  by  different  mechanics 
and  of  different  degrees  of  curvature,  is  very  greatly  in- 
creased. It  is,  however,  not  only  possible  to  bring  two 
dissimilar  lenses  to  absolutely  the  same  focus  without 
having  to  resort  to  regrinding  and  polishing  their  sur- 
faces, but  it  is  a  matter  which  is  not  attended  with  so 
much  difficulty  as  to  be  insurmountable  by  any  reader  of 
intelligence  who  possesses  a  moderate  amount  of  me- 


CRITERION  OF  FOCUS.  155 

chanical  skill.  Let  it,  however,  be  understood  that  the 
two  lenses  must  be  of  a  focus  not  greatly  apart  from 
each  other,  although  too  much  so  to  warrant  their  being 
used  for  binocular  purposes. 

Size  of  Image  the  Criterion  by  which  Focus  is  judged. — 
The  optical  tyro  must  bear  in  mind  the  fact  that  the 
back  focus,  so  called,  of  a  combination,  affords  no  clue 
whatever  to  the  real  focus  of  the  lens.  In  comparing  two 
lenses,  the  size  of  the  image  formed  by  each  is  the  real 
criterion  by  which  they  are  to  be  judged.  There  may 
be  two  lenses  in  which  the  back  elements  of  each  are 
precisely  the  same  distance  from  the  ground  glass  when 
both  are  sharply  focussed,  and  yet  the  size  of  the  re- 
spective images  on  the  ground  glass  be  widely  different. 
The  reason  is  obvious  :  the  equivalent  focus  is  that  by 
which  the  size  of  the  image  is  determined,  and  in  a 
portrait  lens  the  point  from  which  the  equivalent  focus 
is  measured — or  the  focal  centre — has  a  very  wide  range 
of  position,  being  in  some  combinations  near  the  front 
lens,  and  in  others  near  the  back. 

In  combining  two  plano-convex  lenses  of  similar  foci 
— say  of  twelve  inches  each — these,  if  placed  with  their 
flat  sides  in  contact,  will  have  a  focus  practically  of  six 
inches,  and  it  is  not  possible  to  make  of  these  two  any 
shorter  focus  than  this.  But  by  the  mere  expedient  of 
separating  the  lenses,  the  equivalent  focus  may  be 
lengthened  to  the  extent  of  several  inches ;  for  the 
greater  the  distance  between  them — or,  in  other  words, 
the  longer  the  tube  in  which  they  are  mounted — the 
longer  will  be  the  equivalent  focus, 


156  ALTERING  LENSES. 

|  Bearing  this  in  mind  it  becomes  a  very  easy  matter 
to  adjust  a  pair  of  compound  lenses  of  dissimilar  foci,  so 
that  both  shall  produce  images  absolutely  alike  in  respect 
of  size  ;  for  if  one  give  a  smaller  image  than  the  other, 
and  as  by  separating  the  lenses  the  equivalent  focus  is 
lengthened,  a  point  will  be  found  at  which  the  images 
given  by  both  lenses  will  be  similar.  It  may  here  be 
noted  that  in  proportion  as  the  real  focus  is  lengthened 
so  is  the  back  focus  shortened. 

.  In  some  instances  the  difference  between  the  size  of 
the  images  is  so  little  that  both  lenses  may  be  brought 
into  coincidence  by  unscrewing  the  cell  of  the  back  lens 
a  few  turns.  But  if  this  proves  insufficient,  then  should 
there  be  a  short  supplemental  piece  of  tube  screwed  into 
the  principal  tube,  and  into  which  in  turn  is  screwed  the 
cell  containing  the  back  lens. 

What  has  been  here  said  of  portrait  lenses  applies 
equally  to  every  kind  of  combination,  e.g.,  rapid  wide- 
angle  rectilinears  and  symmetricals  ;  and  it  will  be 
obvious  that  the  foci  may  be  assimilated  by  shortening 
the  tube  of  one  lens  as  well  as  by  increasing  the  length 
of  the  other. 

Effect  of  Altering  Lenses  on  duality  of  Image. — There 
are  lenses,  especially  those  of  wide  angular  aperture,  so 
delicately  adjusted  as  regards  the  separation  of  their 
elementary  constituents  as  would  entail  a  degradation 
of  the  definitions  at  the  margin  of  the  picture  by  altering 
them  in  the  way  suggested  ;  but  for  the  purpose  here 
suggested  it  would  be  so  slight  as  to  be  scarcely 
appreciable,  while  it  might  turn  out,  as  it  did  in  one 


FOCUS  RESULTING  FROM  COMBINING  LENSES.  157 

case  under  our  observation,  that  the  performance  of 
the  lens  was  greatly  improved  in  every  respect ;  and, 
at  any  rate  the  original  mount  is  all  the  time  un- 
affected. 

Rule  for  Ascertaining  the  Focus  resulting  from  Combining 
any  two  Lenses. —  It  may  be  well  here  to  give  the  rule 
by  which  the  focus  resulting  from  the  combination  of 
any  two  lenses  of  known  focus  may  be  ascertained. 
Multiply  the  focus  of  one  lens  by  the  other,  and 
divide  this  product  by  the  focus  of  both  added  together, 
less  the  distance  of  separation.  The  quotient  is  the 
focus  sought  for.  Thus — to  take  an  extreme  case  as  an 
example — if  the  two  twelve-inch  lenses  previously  spoken 
of,  and  which  when  in  contact  gave  a  focus  of  six  inches, 
were  mounted  in  a  tube  so  as  to  be  ten  inches  apart,  the 
focus,  instead  of  being  six  inches  as  formerly,  would  now 
be  ten  inches  and  (nearly)  a  quarter.  Again,  having  two 
lenses  respectively  of  twenty  and  twelve  inches  focus, 
and  mounted  two  inches  apart,  what  is  the  equivalent 
focus?  The  answer  may  be  thus  expressed  — 

20  x  12  =       _  2  =  8  inches. 

The  foci  when  added  together  give,  minus  two  (the 
separation),  30,  the  divisor  for  240  (the  product  of  the 
multiplication  of  the  foci)  giving  eight  inches  as  the 
equivalent  focus. 


CHAPTER  XXVII. 

THE  DETERIORATION  OF  LENSES  BY  LIGHT. 

THE  subject  of  the  deterioration  of  lenses  through 
time  or  carelessness  on  the  part  of  assistants  is  one 
fraught  with  much  interest  to  the  photographer,  who 
frequently  has  a  large  amount  of  money  invested  in 
them.  Complaints  as  to  lenses  which  were  at  one  time 
rapid  but  have  become  much  slower  in  action  have  been 
frequent.  In  some  cases  it  is  possible  that  imagination 
has  to  do  with  such  deterioration  ;  but,  for  all  that,  it  is 
not  less  the  case  that  the  falling-off  in  the  effective 
performance  of  a  lens  is  a  physical  fact  which  admits  of 
no  gainsaying. 

Colourless  Glass  a  Factor  in  Rapidity. — The  clearer 
and  more  colourless  is  a  lens  the  better  and  more  rapidly 
does  it  act.  This  may  be  accepted  as  an  axiom  in  pho- 
tography, although  in  astronomical  instruments  and 
microscopic  objectives  it  is  not  of  like  importance.  It  is 
well  known  to  ourselves  and  others  that  of  a  pair  of 
portrait  lenses  which  were  selected,  and  for  some  time 
noted,  for  their  absolute  identity  of  action,  especially  as 
regards  rapidity,  one  afterwards,  which  had  been  for 
over  a  year  relegated  to  a  different  class  of  work  from 


CA  USES  OF  SLO  WNESS.  i $9 

the  other,  eventually  became  so  slow  by  comparison 
with  the  performance  of  its  twin  brother  as  to  prevent 
their  ever  again  being  employed  in  the  capacity  of 
producing  binocular  portraits.  Seeing  that  a  high-class, 
rapid-working  lens  involves  the  expenditure  of  a  con- 
siderable sum,  its  retention  in  a  state  of  pristine  purity 
is,  consequently,  an  object  of  importance. 

Causes  of  Slowness. — There  are  two  sources  of  dete- 
rioration of  a  photographic  objective,  and  we  may  here 
explain  that  by  '  deterioration/  in  the  sense  now  em- 
ployed by  us,  slowness  is  understood.  So  long  ago  as 
the  second  meeting  of  the  London  Photographic  Society, 
held  on  the  3rd  of  March,  1853,  ^  was  we^  recognised 
that  some  lenses  worked  much  slower  than  others  which 
had  similar  dimensions  and  working  aperture,  and  some 
attempt  was  made  to  elucidate  the  cause.  That  the 
yellow  colour  of  the  glass  of  some  of  the  instruments,  as 
contrasted  with  that  of  others,  was  a  prime  factor  in  the 
slowness  was  acknowledged ;  but  it  seemed  to  be  a  moot 
point  as  to  the  part  taken  in  such  degradation  of  work- 
ing by  the  Canada  balsam  with  which  the  component 
parts  of  the  front  lens  were  cemented.  Mr.  Robert 
Hunt,  one  of  the  leading  spirits  of  the  then  young 
Society,  went  so  far  on  the  occasion  referred  to  as  to 
say  that  it  had  been  observed  by  Daguerre  and  others 
that  by  dropping  upon  the  surface  of  a  photographic 
lens  a  little  of  the  purest  oil  of  almonds,  and  then  wiping 
it  off  again  in  as  perfect  a  manner  as  could  be  done  by 
a  silk  handkerchief,  the  attenuated  film  still  left  would 
necessitate  a  great  prolongation  of  the  exposure.  From 


160  CA  USES  OF  SLO  WNESS. 

whatever  cause  it  may  have  arisen,  neither  we  nor  any 
one  whom  we  have  known  to  repeat  this  experiment 
have  found  it  to  yield  the  result  mentioned.  But  that  a 
film  of  Canada  balsam  of  no  great  thickness  will  render 
photographic  action  sluggish  is  a  fact  admitting  of  no 
question.  It  has  been  found  by  Mr.  George  Shadbolt 
that  in  preparing  two  similar  microscopic  objects — the 
parasites  of  birds — for  photographing,  one  of  them  being 
mounted  in  balsam  and  the  other  in  glycerine,  the 
former  required  an  exposure  of  four  minutes,  whereas 
an  equally  good  negative  was  obtained  with  the  latter 
in  one  minute. 

The  great  cause  of  lenses  becoming  slower  is  not  the 
balsam  used  in  cementing  their  elementary  parts  to- 
gether, but  the  discoloration  of  the  glass  itself  by  the 
action  of  light.  Lenses  formed  of  dense  flint  glass  are 
more  liable  to  become  deteriorated  by  the  action  of 
light  than  those  of  light  glass.  Why  this  should  be  so 
we  are  unable  to  say,  although  it  has  been  surmised  that, 
in  some  instances  at  any  rate,  it  may  have  arisen  from  a 
trace  of  silver  present  in  the  lead  which  enters  into  the 
formation  of  flint  glass.  We  well  remember  one  lens  of 
the  '  rapid '  type,  which  was  exhibited  before  the  (now) 
Photographic  Society  of  Great  Britain  several  years  ago, 
by  an  eminent  optician,  as  possessing  a  larger  angular 
aperture,  and  consequently  greater  intensity  of  lighting, 
than  any  lens  of  a  similar  class  ever  previously  produced. 
A  few  years  afterwards,  when  inquiring  of  the  maker 
the  reason  why  a  lens  of  such  obvious  utility  had  not 
been  commercially  manufactured,  he  said  that  the  glass 


CA  USE  OF  D  IS  COL  OR  A  TION.  T  6 1 

of  which  that  specimen  had  been  composed,  and  which 
possessed  a  greater  degree  of  density,  had  deteriorated 
to  such  an  extent  and  become  so  yellow  in  colour  that 
he  would  not  jeopardise  his  reputation  by  allowing  one 
to  be  issued  from  his  establishment.  He  showed  us  the 
lens  in  question,  and  its  yellow  colour  was  quite  noticeable. 

When  Mr.  Thomas  Gaffield,  of  Boston,  brought  the 
subject  of  the  discoloration  of  glass  before  the  British 
Association,  at  the  Brighton  meeting,  in  1872,  and 
showed  examples  of  glass  of  a  fine  quality,  which  from 
being  quite  colourless  had  assumed  a  very  sensible 
degree  of  deterioration  on  being  exposed  to  strong 
sunlight  under  a  mask  for  a  brief  period,  it  was  felt 
that  this  deterioration,  although  of,  perhaps,  primary 
importance  in  such  a  case  as  the  glass  roofing  of  a 
studio,  which  was  constantly  exposed  to  light,  would 
also  affect  photographic  lenses,  in  which  a  degree  of  dis- 
coloration far  less  in  amount  would  produce  a  greater 
effect  in  the  prolongation  of  the  exposure.  To  ascertain 
whether  optical  glass  would  follow  the  role  of  window 
and  plate  glass,  we  wrapped  a  piece  of  tinfoil  round  a 
lens  in  such  a  manner  as  to  allow  one  half  to  be  exposed, 
and  this  we  placed  where  it  could  receive  the  beams  of 
a  September  sun  for  a  protracted  period.  Upon  being 
afterwards  examined  by  laying  it  on  a  sheet  of  white 
paper,  the  exposed  half  caused  the  paper  to  assume  a 
decided  hue  of  a  character  resembling  yellow  with  a 
purplish  tinge. 

Cause  of  Discoloration  of  Glass. — Why  glass  changes 
it  is  not  altogether  easy  to  say  with  certainty.     In  the 

If 


1 62  ACTION  OF  LIGHT  ON  BALSAM. 

case  of  plate-glass  it  is  held  to  arise  from  the  presence 
of  manganese,  which  is  added  in  the  form  of  its  oxide, 
and  known  as  '  glassmakers'  soap.'  One  theory  of  the 
action  of  the  manganese  is  that  in  all  kinds  of  window 
glass,  and  in  some  poorer  sorts  of  flint  glass,  materials 
are  used  which  are  not  chemically  pure.  There  is 
usually  iron  present,  the  protoxide  of  which  imparts  a 
green  colour  to  the  glass.  The  addition  of  the  man- 
ganese causes  some  of  its  oxygen  to  fly  to  the  iron  and 
convert  its  protoxide  into  peroxide,  which  imparts  a 
yellowish  colour  to  the  glass  ;  that,  being  complementary 
to  the  natural  pink  of  the  manganese,  is  neutralised  and 
the  glass  rendered  of  a  white  colour.  By  the  action  of 
sunlight  upon  this  glass  the  nice  equilibrium  between 
the  oxygen  of  the  iron  and  the  manganese  is  disturbed, 
and  sometimes  a  yellow  and  sometimes  a  pinkish  colour 
is  produced.  Another  theory  is  that  the  manganese  is 
added  solely  on  account  of  the  facility  with  which  it 
parts  with  oxygen,  which  consumes  any  impurities  of 
an  organic  character  or  any  oxidised,  opaque,  metallic 
particles.  A  singular  fact  in  connexion  with  the  dis- 
coloration of  glass  by  the  action  of  light  is  found  in 
the  further  fact  that  by  heating  glass  thus  deteriorated 
it  becomes  decolorised. 

Action  of  Light  on  Canada  Balsam. — Now  at  this  stage 
an  element  imagined  to  be  of  a  conflicting  nature  has 
to  be  introduced  ;  it  is  the  Canada  balsam.  Painters 
are  aware  that  white  oil  paint  (carbonate  of  lead)  when 
mixed  with  megilp,  although  pure  enough  while  it 
remains  exposed  to  light,  assumes  quite  a  yellow 


ACTION  OF  LIGHT  ON  GLASS.  163 

appearance  upon  being  kept  in  darkness,  or,  in  the 
case  of  a  painting,  in  a  drawer  for  a  few  months  or 
even  weeks.  In  like  manner  it  is  affirmed  that  Canada 
balsam  becomes  bleached  and  colourless  by  the  action 
of  light,  resuming  its  yellow  appearance  when  kept  in 
the  dark.  Here,  then,  are  two  antagonistic  forces  to  be 
kept  under  check.  If  the  lens  be  exposed  to  strong 
light  the  glass  has  a  chance  of  being  discoloured  while 
the  balsam  becomes  decolorised  ;  but  if  the  lens  be 
kept  in  darkness  (except  when  in  active  use)  the  glass 
remains  pure,  while  the  balsam  becomes  discoloured. 
Now,  while  it  is  true  that  the  discoloured  white  of  the 
megilp  oil  painting  will  assume  its  original  purity  when 
placed  in  the  sun  for  a  few  hours  (unless  it  be  a  very  bad 
case  indeed),  and,  further,  that  coloured  balsam  will  also 
become  colourless,  it  is  not  the  case  that  every  kind  of 
balsam  changes  colour  ;  and  we  believe  we  speak  within 
the  mark  in  saying  that  for  the  productions  of  one 
optician  which  become  deteriorated  on  this  ground, 
those  of  twenty  are  unaffected.  The  balsam  scare, 
therefore,  need  not  prove  a  source  of  uneasiness  to 
photographers,  the  more  especially  as  by  the  means 
we  recently  indicated  the  old  balsam  may  be  readily 
cleaned  away  and  its  place  supplied  with  a  fresh  and 
colourless  sample. 

Strong  Light  Discolours  Lenses. — Of  much  greater  im- 
portance is  it  that  the  lens  be  not  subjected  to  any 
strong  light,  as  it  may  cause  a  discoloration  in  the 
substance  of  the  glass  that  cannot  be  removed.  We 
do  not  hepe  allude  to  surface  stains  in  the  form  of 


1 64  ACTION  OF  LIGHT. 

oxidised  patches,  which  are  often  caused  by  damp  and 
particles  of  dirt  acting  as  nuclei,  and  which  stains  are 
capable  of  being  polished  out,  but  to  a  discoloration 
existing  throughout  the  entire  substance  of  the  glass. 

If  an  objective  be  employed  for  forming  an  image 
by  the  direct  beams  from  the  sun,  such  as  is  used  in  the 
solar  camera,  we  advise  that  it  be  kept  for  that  purpose 
exclusively,  because  of  the  facilities  which  the  light  has 
for  acting  injuriously  upon  it  and  rendering  it  slower. 
We  would  also  state  that  of  all  classes  of  lenses  which 
should  not  be  employed  in  the  solar  camera,  or  for  any 
other  purpose  associated  with  the  transmission  of  bright 
light,  those  of  the  popular  'rapid'  type  stand  at  the 
head  ;  for  being  formed  of  dense  glass  they  are  more 
liable  than  any  others  to  undergo  change.  It  is  well, 
therefore,  to  keep  them  covered  as  much  as  possible 
when  not  in  use.  Portrait  combinations  and  ordinary 
single  achromatic  landscape  lenses,  being  formed  of 
glass  of  less  density,  are  better  able  to  resist  the  in- 
fluence of  light ;  but  even  these  should  always  have 
their  caps  replaced  after  being  used. 


CHAPTER  XXVIII. 

HOW  TO  ASCERTAIN  THE  ANGLE  OF  VIEW  INCLUDED 
BY  ANY  LENS. 

LET  it  be  first  of  all  understood  that  every  class  of  lens 
having  the  same  focus  and  covering  power  embraces  the 
same  angle  on  a  plate  of  a  given  size.  It  is  of  no  conse- 
quence what  is  the  nature  of  the  lens,  or  by  what  name  it 
is  called,  whether  single  landscape,  wide  angle  or  narrow 
angle,  rectilinear,  symmetrical  or  portrait  lens — one  thing 
is  true  of  them  all — that  if  they  be  of  similar  foci  the 
angle  subtended  on  a  plate  of  a  certain  number  of 
inches  will  be  alike  in  all  of  them.  It  is  the  focus  of 
the  lens  and  that  alone  which  determines  the  angle 
of  picture  depicted  on  a  plate  of  any  given  size.  Some 
lenses  may  work  sharper  or  quicker  than  others  ;  but, 
though  a  mere  simple  spectacle  glass  be  used,  or  even 
in  the  absence  altogether  of  a  lens,  a  small  hole  in  front 
of  the  camera  be  employed  for  producing  the  image,  the 
rule  holds  good. 

In  lenses  of  the  distorting  kind — such  as  the  ordinary 
single  combination  with  a  stop  in  front — the  compression 
of  objects  in  proportion  as  they  recede  from  the  centre 
of  the  picture  apparently  militates  against  the  accuracy 


i66  WIDE-ANGLE  LENSES  SLOW. 

of  the  rule  here  laid  down  ;  but  the  difference  in  reality 
is  so  slight  as  not  to  demand  attention,  and  the  ad- 
vantage they  possess  in  this  respect  over  the  non- 
distorting  class  of  lenses  may  in  practice  be  ignored. 

Wide-Angle  Lens  used  for  Narrow- Angle  Views.— Let  us 
narrow  this  question  and  apply  it  to  special  cases.  Here 
are  two  landscape  objectives  of  equal  foci,  but  one  is  a 
wide-angle  and  the  other  a  narrow-angle  lens.  If  the 
latter  cover  a  12x10  plate  and  the  former  an  iSx  15 
plate,  but  for  various  reasons  the  wide-angle  one  be 
only  used  for  a  12x10,  will  there  be  any  difference 
between  the  productions  of  the  wide  and  the  narrow 
angle  lenses  ?  Certainly  not.  Is  there  then  any  ad- 
vantage in  having  a  wide-angle  lens  for  such  a  purpose 
as  that  in  question  ?  None  whatever.  But  there  is  this 
advantage,  that  although  it  can  do  all  the  work  that  is 
done  by  the  narrow-angle  instrument  (at  the  expense, 
however,  of  rapidity — for  every  gain  is  attended  by  a 
loss  in  some  other  direction),  it  can  do  more  if  required. 
It  can  be  used  on  an  18x15  plate,  whereas  the  other 
cannot. 

Wide -Angle  Lenses  Necessarily  Slow.— We  have  spoken 
of  a  loss  of  rapidity  in  connexion  with  lenses  of  wide 
angle.  This  is  inseparable  from  their  method  of  con- 
struction, their  curves  being  deeper  than  those  of  the 
narrow  class,  and  necessitating  the  employment  of  a 
smaller  stop,  for  the  narrower  the  angle  sought  to  be 
included  by  a  lens  the  greater  may  be  its  aperture  in 
proportion  to  its  focus  and  vice  versa. 

How  to  Measure  the  Angle  of  View. — The  question  ma> 


MEASURING  ANGLE  OF  VIEW.  167 

now  be  asked — By  what  means  can  it  be  ascertained 
what  angle  of  view  a  lens  includes  on  a  plate  of  any 
certain  size?  Before  answering  this  we  may  make 
what  to  the  majority  of  readers  will  be  a  self-evident 
statement :  if  a  lens  include  on  a  plate  of  twenty  inches 
diagonal  an  angle  of  view  of  40°,  on  a  plate  of  ten 
inches  diagonal  the  included  angle  will  only  be  20°. 

Draw  on  a  sheet  of  paper  of  sufficient  size  a  straight 
line  equal  to  the  diagonal  of  the  plate  on  which  the 
negative  is  taken,  say  \^\  inches  for  a  12x10  plate 
for  exarnple,  and  from  the  centre  of  this  line  erect 
a  vertical'  line  equal  in  length  to  the  focus  of  the  lens. 
In  the  diagram  the  line  A  B  is  made  the  length  of  the 

c 

\ 


a 


d 

FIG.  49. 


,  diagonal,  whatever  that  may  be,  and  the  line  C  D  is  that 
which  in  a  corresponding  manner  is  made  of  the  length 
of  the  focus  of  the  lens.  Now  with  a  pencil  draw  lines 
from  C  (the  lens)  to  A  and  B  (two  corners  of  the  plate), 
and  the  angle  thus  made  with  the  pencil,  or  A  C  B, 
represents  the  angle  of  view  included, 


168  PROTRACTOR. 

There  are  few  cases  of  drawing  instruments  sold  in 
which  there  is  not  in  some  form  or  other  a  protractor  to 
be  found  by  which  angles  may  be  measured.  But  some 
photographers  may  perhaps  not  have  access  to  such  an 
instrument,  so  we  will  now  describe  how  such  may  make 
for  themselves  a  protractor  which  if  not  so  elaborate  as 
those  sold  by  the  dealer  in  mathematical  instruments 
will  yet  be  as  useful  as  the  best  of  them,  and  possibly 
be  more  easy  to  employ.  The  following  diagram  repre- 


50. 


sents  a  protractor  which  includes  an  angle  of  90°,  and 
divided  into  nine  equal  parts,  each  part  including  ten 
degrees.  These  are  further  subdivided  so  as  to  permit 
five  degrees  to  be  read  off.  See  Fig.  50. 

To  use  this  protractor  :  having  laid  down  a  line  eo^ual 


PROTRACTOR.  169 

to  the  diagonal  of  the  plate,  having  further  erected  the 
centre  perpendicular  line  equal  in  length  to  the  focus  of 
the  lens,  and  having  also  drawn  the  pencil  angle  lines 
already  described,  place  the  protractor  down  upon  the 
lines  thus  drawn,  the  point  Z  being  placed  exactly  on 
the  point  of  the  vertical  line,  and  let  the  line  Z  S 
coincide  with  that  drawn  from  C  to  A  in  Fig.  49. 
Observe  now  on  what  part  of  the  protractor  the  other 
boundary  line  (that  from  C  to  B)  falls,  and  the  figures 
indicate  the  angle  sought  for.  No  calculations  are 
required,  and  the  results  are  obtained  in  the  simplest 
manner. 


CHAPTER   XXIX. 

REFINED   FOCUSSING   BY    MEANS   OF   A   TELESCOPE. 

SOMETIMES  occasions  arise  in  which  it  is  necessary 
to  focus  with  extreme  sharpness,  even  without  a  focus- 
ing screen. 

At  the  Derby  Convention  in  1886,  the  author  ex- 
hibited a  camera  to  which  was  attached  a  pocket  tele- 
scope to  ensure  absolute  sharpness,  and  the  conditions 
for  the  using  of  which  we  shall  discuss. 

Ground  Glass  Screens  inadequate  for  absolute  Focussing. 
— If  the  acme  of  perfection  in  focussing  be  desired,  the 
image  should  be  an  aerial  one,  that  is,  not  broken  up  by 
being  projected  upon  ground  glass  which  renders  it 
difficult,  if  not  altogether  impossible,  for  any  one  to  see 
it  distinctly  when  employing  a  high  magnifying  power 
for  such  purpose.  Just  imagine  the  case  if  in  a  tele- 
scope a  ground  glass,  no  matter  how  fine  its  surface, 
were  interposed  between  the  eye-piece  and  the  object- 
glass  at  the  point  of  focus.  The  system  of  focussing 
now  to  be  advocated  and  described  permits  of  the  dark 
slide  being  inserted  into  its  place  in  the  camera,  its 
shutter  drawn,  and  everything  in  readiness  for  the  final 
uncapping  of  the  lens,  and  all  this  without  having  deter- 
mined upon  the  precise  object  at  which  the  shot  is  to  be 
made,  or  its  distance  from  the  camera,  which  in  this 


AERIAL  IMAGES.  171 

case  may  have  a  lens  of  twenty,  thirty,  or  even  forty 
inches  focus,  and  be  practically  wanting  in  what  is 
known  as  depth,  and  which  entails  the  necessity  of 
adjusting  the  focus  upon  the  definite  object  to  be  taken, 
and  not  upon  one  either  nearer  or  farther  away. 

Aerial  Images. — A  well-corrected  lens,  when  directed 
to  any  scene,  produces  at  its  focus  an  aerial  model  of 
that  scene,  each  portion  of  which  presents  the  same 
relative  distance  to  or  from  any  other  as  do  the  same 
portions  of  the  original.  In  a  lens  of  short  focus  the 
whole  of  this  aerial  model  is  on  a  scale  so  diminutive 
and  compressed  that,  except  such  portions  as  are  close 
at  hand,  the  distance  relations  between  the  others  is  too 
close  to  enable  the  eye  to  distinguish  easily  between 
them,  and  hence  we  say  that  everything  beyond  a 
certain  distance  is  in  equal  sharpness,  this  '  certain  dis- 
tance '  being  nearer  to  the  lens  the  shorter  is  its  focus  ; 
but,  conversely,  the  longer  is  the  focus  of  the  lens  the 
greater  is  the  separation  of  the  component  parts  of  the 
subject  that  is  being  examined,  and  the  farther  is  that 
distance  beyond  which  everything  is  practically  simul- 
taneously sharp.  Five  miles  is  a  fairly  long  distance 
away,  and  so  for  that  matter  is  one  mile ;  but  let  an 
object  at  the  greater  distance  be  examined  through  a 
large  telescope,  the  focus  of  which  has  been  set  for 
looking  at  something  only  one  mile  off,  and  it  will  be 
seen  quite  indistinctly  until  refocussing  has  been  had 
recourse  to ;  and  when  the  five-mile  object  is  made  sharp 
a  more  distant  object  still  will  be  blurry  and  indistinct 
until  it  in  turn  has  been  sharpened  by  refocussing. 


r 

172  A  FOCUSSING  TELESCOPE. 

Nature  of  a  Focussing  Telescope. — Let  a  little  pocket 
telescope  be  procured,  the  object-glass  of  which  is  the 
same  focus  as  that  of  the  lens  on  the  camera  ;  such  a 
telescope  costs  but  little,  and  quite  apart  from  the  special 
use  for  it  which  is  about  to  be  described,  it  forms  a 
most  useful  companion  when  one  is  away  from  home. 
One  of  such  dimensions,  with  three  draws  and  a  leather- 
covered  body,  as  will  suit  a  camera  of  the  average  class 
employed  in  taking  views  on  plates  ten  or  twelve  inches 
in  size,  can  readily  be  obtained  at  a  price  under  twice  as 
many  shillings,  for  high-class  workmanship  is  not  neces- 
sary ;  what  is  of  importance  is  that  the  focus  of  the 
telescopic  object-glass  and  that  of  the  photographic  lens 
must  be  the  same.  To  prepare  this  telescope  for  camera 
use  it  is  only  necessary  that  one  of  the  draws  be  made 
so  easy  as  to  slide  in  or  out  by  a  touch.  The  one  most 
convenient  for  this  is  the  second  from  the  eye-piece  end, 
and  the  requisite  ease  in  drawing  can  be  imparted  by 
unscrewing  that  particular  tube  and  scraping  the  interior 
of  the  short  piece  into  which  it  travels,  or  by  bending 
out  the  slots  usually  made  in  it  to  give  it  a  springy 
smoothness  of  motion. 

How  to  attach  the  Telescope  to  Camera. — On  the  top  of 
the  camera  front  to  which  the  lens  is  attached,  or  from 
its  side  (it  is  immaterial  which,  so  long  as  it  does  not 
interfere  with  other  movements),  projects  a  pin,  on  which 
the  telescope  fits  by  means  of  a  small  hole  cut  into  the 
leathered-covered  portion  of  the  body  at  any  convenient 
distance  from  the  object-glass  end.  In  the  one  shown 
a,t  Derby  this  distance  is  three  inches  from  it.  The 


FITTING  TELESCOPE  TO  CAMERA.         t^ 

same  thing  must  be  done  with  the  eye-piece  end  of  the 
telescope,  the  second  sliding  tube  of  which,  by  preference, 
must  be  connected  in  a  similar  way  with  the  frame  of 
the  camera  which  carries  the  dark  slide.  It  is  of  no 
consequence  whatever  how  or  where  the  outer  end  of 
the  telescope  is  attached  to  the  lens  end  of  the  camera, 
but  care  is  required  in  determining  the  fixing  of  the 
other.  It  is  effected  in  this  way :  Focus  the  camera 
lens  on  the  ground  glass  on  any  moderately  distant 
object  with  the  greatest  care,  using  a  magnifying  glass 
for  this  purpose,  and  noting  the  object  that  is  in  the 
centre  of  the  field.  Then,  stepping  the  telescope  on  the 
pin  in  the  front  of  the  camera,  direct  it  to  the  object 
forming  the  centre  of  the  scene  on  the  ground  glass, 
and,  having  pulled  out  the  eye-piece  tube  to  its  limit, 
focus  sharply  by  means  of  the  second  tube,  into  which 
the  second  small  hole  has  been  drilled,  and  which  will, 
or  ought  to,  fall  on  some  solid  portion  of  the  body  or 
frame  which  receives  the  dark  slide.  Now  insert  the 
pin  so  that  the  expansion  of  the  telescope  is  fixed  at 
that  point,  and  all  the  fitting  is  accomplished.  It  will 
now  be  found  that,  by  racking  the  camera  in  or  out,  the 
telescope  body  will  also  slide  with  facility. 

To  Use  this  System. — We  shall  suppose  that  the  ob- 
ject to  be  photographed  is  a  ship  rapidly  proceeding  out 
to  sea,  but  that,  owing  to  lighting  or  any  other  contin- 
gency, the  precise  moment  for  effecting  the  exposure  is 
uncertain,  and  that  the  distance  between  ship  and 
camera  is  ever  increasing  (or  lessening).  To  watch  the 
motions  of  the  ship  upon  the  ground  glass  would  be 


174  APPLICA  T10N  TO  LENSES. 

preposterous,  because,  when  the  proper  moment  for 
exposure  arrived,  the  time  occupied  in  removing  the 
focussing  screen  and  getting  the  dark  slide  inserted 
might  cause  a  delay  which  would  prove  fatal  to  ob- 
taining the  right  effect  at  the  right  instant,  whereas, 
without  the  ground  glass  examination,  one  could  not  be 
quite  certain  of  the  object  being  in  correct  focus.  But 
by  the  telescopic  system,  all  that  is  necessary  is  to  in- 
sert the  dark  slide  and  let  the  plate  remain  open,  subject 
to  the  operation  of  the  instantaneous  shutter,  watch  the 
ship  through  the  telescope,  which  is  kept  in  sharp  focus 
by  the  rack  and  pinion  of  the  camera,  and  at  the  fitting 
moment  press  the  pneumatic  ball  of  the  shutter,  when 
the  image  will  be  secured  with  a  degree  of  facility  and 
accuracy  of  focus  quite  incapable  of  being  attained  in 
the  usual  way. 

Application  to  Lenses  of  Various  Foci. — The  real  use  of 
this  system  is  to  be  found  when  employing  lenses  of 
long  focus  and  rather  large  aperture,  but  for  experi- 
mental purposes  we  have  also  had  one  attached  to  a  small 
camera  in  which  we  use  lenses  varying  in  focus  from  five 
to  eight  inches,  and  in  accordance  with  an  optical  law 
we  have  made  the  object-glass  of  the  little  telescope 
adaptable  for  all  lenses  ranging  between  these  foci.  The 
law  referred  to  is  treated  of  in  the  chapter  (page  154) 
'  On  the  Adjustment  of  Dissimilar  Lenses,'  but  may  here 
be  summarised  as  follows  :  When  two  lenses  of,  say,  ten 
inches  each  in  focus  are  placed  in  contiguity,  the  focus 
is  reduced  to  five  inches  approximately,  but  in  pro- 
portion as  they  are  separated  so  does  the  focus  become 


APPLICATION  TO  LENSES.  175 

lengthened.  Hence,  by  having  two  object-glasses  of 
ong  focus  each  instead  of  one  in  the  telescope,  the  inner 
one  being  in  a  small  travelling  tube  moving  inside,  and 
capable  of  being  run  pretty  close  up  towards  the  eye- 
piece, a  considerable  range,  or  rather  variety,  of  foci  is 
obtained,  the  precise  amount  of  focal  power  being  deter* 
mined  by  graduations  at  the  side  of  the  slot  through 
which  the  button  projects  by  which  the  inner  runner  is 
moved.  By  a  camera  fitted  with  a  little  telescope  of  the 
nature  described,  we  have,  with  a  lens  working  with  an 
aperture  of /-4,  selected  one  or  more  boys  among  groups 
which  were  playing  at  cricket  on  a  common,  and  by 
following  them  with  the  telescope,  focussing  all  the 
while  by  the  camera  rack,  we  have  been  able  to  '  snap  ' 
them  off  in  individual  sharpness,  while,  owing  to  the 
unusually  large  aperture  of  the  lens,  all  their  surround- 
ings were  more  or  less  out  of  focus.  But  many  applica- 
tions of  the  system  will,  doubtless,  suggest  themselves  to 
the  ingenious  reader. 

Although  we  have  never  experienced  any  difficulty 
in  procuring  little  telescopic  object-glasses  of  any  desired 
focus,  yet  it  is  conceivable  that  those  at  a  distance  from 
centres  of  optical  industry  may  not  be  equally  fortunate, 
Such  may  be  interested  in  learning  that  in  the  case  of 
an  uncemented  achromatic  object-glass  of  a  cheap  tele- 
scope (which  are  almost  invariably  uncemented)  a  dif- 
ference in  the  focus  results  by  the  insertion  of  a  ring 
between  the  flint  and  crown  lenses  so  as  to  separate 
them.  The  concave  lens  of  the  combination  being 
nearer  to  the  eye-piece  than  the  convex  or  crown-glass 


W  APPLICATION  TO  LENSES. 

one,  the  farther  apart  they  are  separated  the  shorter  will 
be  their  focus,  being  in  this  respect  contrary  to  the  effect 
produced  if  both  lenses  were  positive,  as  previously 
explained. 

If  a  telescope  with  two  achromatic  object-glasses  be 
desired  so  as  to  permit,  as  in  a  case  cited,  of  its  being 
made  to  suit  a  camera  to  which  more  than  one  lens  of  a 
certain  focus  is  to  be  affixed,  the  rule  by  which  any 
definite  focus  of  such  telescopic  objective  may  be  accu- 
rately determined  or  ascertained  is  the  same  as  that  in 
the  chapter  just  referred  to,  viz.  :  Knowing  the  focus  of 
each  of  the  two  object-glasses,  add  them  together,  and 
subtract  the  distance  of  their  separation  ;  then  multiply 
the  two  foci  together  and  divide  this  last  quantity  by 
the  first,  which  gives  the  precise  focus  of  the  two  lenses 
when  combined  ;  the  focus  thus  can  be  lengthened  by 
increasing  the  separation,  and  by  the  above  rule  this  can 
be  done  with  unerring  accuracy. 


CHAPTER   XXX. 

ANASTIGMATIC   LENSES. 

OF  the  many  lenses  described  in  the  preceding 
pages,  Petzval's  Portrait  Combination  and  Dr.  Adolf 
Steinheil's  Aplanats  were,  doubtless,  the  most  useful 
types,  but  their  definition  of  the  oblique  pencils 
was  marred  by  astigmatism.  If  the  defining  power  of 
an  aplanat  be  examined,  as  described  on  p.  139,  it  will 
be  found  impossible  to  focus  simultaneously,  near  the 
margin  of  the  plate,  lines  drawn  at  right  angles  to  each 
other.  The  images  of  the  two  sets  lie  in  different  planes, 
which  may  be  seen  by  altering  the  distance  between  lens 
and  screen  until  sharp  definition  is  obtained.  At  a  short 
distance  from  the  centre  of  the  field  one  set  is  fuzzy, 
when  the  other  is  sharply  defined.  This  is  the  defect 
known  as  astigmatism,  and  the  lens  from  which  it  has 
been  eliminated  is  called  an  anastigmat.  Many  years 
ago  opticians  came  to  the  conclusion  that  the  error  could 
not  be  corrected  until  the  glass-maker  could  supply 
glass  possessing  suitable  properties.  In  a  report  upon 
the  scientific  apparatus  shown  at  the  International 
Exhibition  held  in  London  in  1876,  Professor  Abbe 
emphasized  that  there  was  little  hope  of  progress  in 
optical  instruments  until  new  varieties  of  glass  were 
manufactured.  This  document,  which  was  published  in 

N 


i ;8  ANASTIGMATIC  LENSES. 

1878,  came  under  the  notice  of  Dr.  Schott,  son  of  a 
glass  manufacturer  at  Witten,  Westphalia.  He  com- 
municated with  Professor  Abbe,  and  it  was  agreed  that 
they  should  mutually  endeavour  to  solve  the  problem, 
Dr.  Schott  making  experimental  specimens  and  Pro- 
fessor Abbe  determining  their  optical  qualities.  Some 
very  remarkable  samples  of  glass  were  made,  and 
these  results  were  thought  to  be  so  important  that  the 
Prussian  Government  was  induced  to  offer  a  subsidy  to 
enable  the  firm  of  Schott  and  Genossen  to  experiment 
on  a  scale  sufficiently  large  for  commercial  purposes.  A 
number  of  new  varieties  of  glass  were  placed  upon  the 
market  in  1886,  and  many  others  have  since  been  added 
to  the  list.  Those  which  differ  materially  from  the  kinds 
previously  made  are  usually  called  new  Jena  glasses, 
after  the  German  town  where  the  factory  is  situate.  In 
the  older  varieties  the  dispersion  of  colour  increases  with 
the  refraction,  but  among  the  new  there  are  deviations. 
Heaviest  Barium  Crown,  for  instance,  combines  high  re- 
fraction with  low  dispersion.  A  new  era  in  photographic 
optics  has  thus  been  opened,  and  it  is  surprising  how 
many  new  combinations  of  lenses  have  been  invented. 

There  are  two  kinds  of  cemented  achromatic  lenses 
which  have  been  found  of  great  use  in  the  construction 
of  anastigmats.  Otto  Lummer  has  named  them  Old 
and  New7  Achromats.  As  the  old  definitions  of  Crown 
and  Flint  do  not  apply  to  some  of  the  Jena  glasses,  Dr. 
Rudolph  defines  '  Flint,'  as  the  glass  with  higher  relative 
dispersion,  and  '  Crown  '  as  that  with  lower  relative  dis- 
persion. For  the  construction  of  an  anastigmat  with 


ANASTIGMATTC  LENSES.  179 

cemented  surfaces  it  is  necessary  to  combine  a  new  with 
an  old  Achromat,  for  the  reasons  given  in  the  following 
propositions  formulated  by  Dr.  Rudolph  :— 

'  To  correct  spherical  aberration  in  a  cemented 
system,  a  Crown  of  lower  refractive  index  than  the  Flint 
must  be  used '  (Old  Achromat). 

*  To  correct  astigmatism  in  a  cemented  system,  a 
Crown  of  higher  refractive  index  than  the  Flint  must  be 
used '  (New  Achromat). 

Dr.  Rudolf  Steinheil  published  in  Eder's  Jahrbuch 
filr  Photographic  und  Reproditctionstechnik,  1897,  an 
article  on  ihe  '  Origin  and  History  of  the  Orthostigmats,' 
which  defines  in  broader  terms  the  necessary  conditions 
for  correcting  spherical  aberration  and  astigmatism.  We 
give  these  dicta  also,  as  they  will  be  found  applicable  to 
the  more  recent  anastigmats  in  which  air  spaces  are 
used  : — 

'  An  objective  can  be  corrected  for  spherical  aberration 
if  two  media  are  separated  by  a  convex 
surface  turned  towards   the   medium  of 
higher  refraction  '  (Old  Achromat). 

'An  objective  can  be  corrected  for 
astigmatism  if  two  media  are  separated 
by  a  concave  surface  turned  towards  the 
medium  of  higher  refraction  '  (New 
Achromat). 
FIG  ci  ^£*  51  rePresents  an  Old  Achromat,  FIG-  52- 

Fig.  52  a  New  Achromat. 

The  flints  are  indicated  by  A  and  C,  and  the  crowns 
by  B  and  D, 


1 80  A  NA  S  TIGMA  TIC  LENSES. 

As  B  is  a  crown  of  lower  refraction  than  the  flint  A, 
we  may  infer  that  the  combination,  Fig  5  I,  is  corrected  for 
spherical  aberration,  also  that  astigmatism  is  corrected 
in  Pig.  52,  because  I)  is  a  crown  with  higher  refraction 
than  the  flint  a 

By  examining  the  nature  of  the  contacts,  similar 
inferences  can  be  drawn  from  the  propositions  laid  down 
by  Dr.  Steinheil.  B  presents  a  convex  surface  to  A,  the 
medium  of  higher  refraction,  and  corrects  spherical 
aberration.  C  presents  a  concave  surface  to  I),  the 
medium  of  higher  refraction,  and  thus  corrects  astigma- 
tism. 

The  use  of  glass  with  high  refraction  and  low  dis- 
persion permits  of  many  new  combinations  of  lenses. 
The  optician's  skill  is  shewn  by  the  invention  of  new 
combinations,  the  selection  of  the  most  suitable  kinds 
of  glass  for  their  construction,  and  the  calculation  of 
the  best  curves,  thicknesses,  and  distances  of  separation. 

Some  residual  errors  had  to  be  remedied  in  the  older 
lenses  by  using  a  stop.  They  were  especially  apparent 
at  the  margin  of  the  picture,  but  have  been  almost 
eliminated  from  the  best  anastigmats,  and  thus  the 
usefulness  of  the  camera  has  been  largely  increased. 
This  is  notably  the  case  with  apparatus  of  small  size,  as 
the  shorter  focus  of  the  lens  gives  the  necessary  depth 
of  definition. 

In  the  drawings  of  the  lenses  described  in  this 
chapter  the  kinds  of  glass  are  indicated  by  the  shading, 
according  to  the  method  used  by  Dr.  Moritz  von  Rohr 
in  his  work,  Theorie  und  Geschichte  des  photographischen 


ANASTIGMATIC  LENSES.  181 

Objectivs,    to    which    I     am    also    indebted    for     many 
particulars. 

Flint   glass    is    indicated    by    lines    leaning    to    the 

right  :— x 
53  ^j^ 

Old,  low  refracting  crown,   by   lines   leaning  to  the 
left  :—  ^ 


New,  high   refracting  crown,  by  horizontal  lines  :-— 

Air  spaces  are  left  blank. 

The  light  is  assumed  to  pass   through  the  lens  from 
left  to  right. 

The  Antiplanet,  shown  on  page  72,  may  be  regarded 
as  the  forerunner  of  the  anastigmats.  It  was  invented 
by  Dr.  Adolf  Steinheil  about  1879,  and  its  great 
originality  of  construction  entitled  it  to  more  attention 
than  it  received  in  England.  Only  the  old  varieties  of 
glass  could  be  had,  yet,  despite  this  disadvantage,  the 
correction  for  astigmatism  is  improved,  if  the  lens  be 
compared  with  its  antecedent,  the  Aplanat.  The  biconvex 
of  the  front  and  the  biconcave  of  the  back  combination 
are  of  flint.  The  biconcave  of  the  front,  and  the  bicon- 
vex of  the  back  combination  are  of  ordinary  crown m 
His  intention  evidently  was  to  construct  a  lens  con- 
forming, as  closely  as  circumstances  permitted,  to  the 
conditions  subsequently  published  by  his  son,  Dr.  Rudolf 
Steinheil.  It  will  be  seen  in  the  front  combination  that 
the  negative  crown  element  presents  a  concave  surface 
to  the  more  refractive  flint,  and  that  the  positive  crown 
in  the  back  combination  presents  a  convex  surface  to 


1 82  ANASTIGMA  TIC  LENSES. 

the  more  refractive  flint  But  with  the  restricted  choice 
of  glass,  this  was  only  possible  by  making  one  com- 
bination negative,  the  other  positive,  and  compensating 
the  errors  in  one  by  opposite  errors  in  the  other. 

The  '  Concentric '  was  the  next  objective,  in  which 
correction  of  astigmatism  was  the  leading  feature.  It 
was  patented  by  Dr.  Hugo  Schroeder  and  Mr.  John 
Stuart  in  1888  and  was  the  first  pho- 
tographic lens  in  which  a  new  Jena 
glass  was  used.  Fig.  53  shows  its 
construction.  The  front  and  back 
surfaces  of  each  achromatic  pair  are 
struck  from  a  common  centre.  The 

HU.    53. 

focus  would  be  negative  were  only 
one  kind  of  glass  used,  but  as  the  plano-convex  of  high 
refraction  and  low  dispersion  is  cemented  to  a  plano- 
concave of  lower  refraction  and  equal  or  higher  disper- 
sion, an  achromatic  lens  of  positive  focus  is  formed.  To 
obtain  a  flat  image  throughout  a  large  field  of  view,  the 
radii  of  the  external  surfaces  are  given  a  certain  ratio  to 
each  other,  dependent  upon  the  refraction  and  dispersion 
of  the  two  kinds  of  glass.  It  is  found  in  practice  that  the 
limits  of  the  refractive  indices  for  the  plano-convex  are 
from  about  159  to  r6i,  and  for  the  plano-concave  from 
about  i'5o  to  i '5 3,  taking  the  D  line  of  the  spectrum. 
Spherical  aberration  is  not  corrected  for  the  full  aper- 
ture, which  is  a  serious  drawback,  as  the  necessary 
stop  reduces  the  intensity  of  the  lens  considerably. 
Had  the  lens  been  brought  out  sooner,  it  might  have 
enjoyed  more  popularity,  but  the  advent  of  the  Zeiss 


ANASTIGMATIC  LENSES. 


.83 


Protars,   not  long  afterwards,  destroyed  any  chance  of 
its  success. 

The  triplet  planned  by  Professor  Abbe  and  calculated 
by  Dr.  Rudolph  was  the  first  of  the  Zeiss  photographic 
lenses,  the  date  of  its  introduction  being  1889.  Fig.  54 
shows  the  construction.  The  outer  elements  are  of 
miniscus  form  and  their  spherical  and  chromatic  errors 
are  corrected  by  a  compound  lens  placed  in  the  centre 


FIG.    54- 

of  the  combination.  This  corrector  is  formed  of  a  double 
convex  borate  crown  element,  cemented  between  two 
negatives.  The  spherical  correction  is  very  good,  but 
the  lens  is  as  astigmatic  as  the  Aplanat.  It  is  included 
in  this  chapter  for  convenience,  but  should  not  be 
regarded  as  an  anastigmat.  Professor  Abbe  aimed 
rather  at  apochromatism.  The  aperture  of  the  lens  is 
/-i6. 

Dr.  Schroeder  put  in  a  claim  for  prior  invention  of 
this  type,  having  published,  in  the  AstronomiscJie 
Nachrichten,  the  description  of  an  objective  specially 
adapted  for  celestial  photography,  which  is  practically 


1 84 


ANAS  TIG  MA  TIC 


the    same.     Dr.    Schroeder    tried    not   only    triple,    but 

double  and  quadruple  combinations,  for  the  construction 

of  the  central  dialyte. 

In  1890  a  patent  was  granted  to  Dr.  P.  Rudolph  for 

a  series  of  lenses,  which  are  now  known  as  '  Protars.' 
They  have  been  issued  in  seven 
different  degrees  of  rapidity. 
This  was  the  first  successful 
attempt  to  supply  photographers 
with  an  achromatic  objective 
combining  spherical  correction 
with  anastigmatic  flatness  of 
field.  In  all  the  series  the  front 
combination  is  an  old  achromat. 
The  back  is  a  new  achromat, 
or  a  more  complex  construction 

equivalent  to  a  new  achromat.     Fig.   55   represents  the 

simplest  form  of  the  Protar.    By  referring  to  page  69,  it 

will  be  seen  how  near  Thomas 

Ross  and  Thomas  Grubb  came 

to  this  type,  but  the  crown  glass 

necessary    for    the    construction 

of  a  new  achromat  did  not  then 

exist. 

The    two    combinations     of 

which  the  simple  series  of  Pro- 
tars    are     constructed     will     be  6 

recognised  by  the  shading  of  the 

glass.      The    front    corrects    spherical    aberration    and 

the    back    astigmatism.       Both    are    of    positive    focus 


ANASTIGMATIC  LENSES. 


,85 


and  approximately  corrected  for  colour.  Fig.  56  shows 
one  of  the  more  complex  series  of  Protars.  The  back 
combination  consists  of  three  elements,  of  which  the 
two  outer  are  positives  of  higher  refraction  than  the  flint 
they  enclose.  The  advantages  thus  secured  are  better 
correction  of  astigmatism  and  greater  flatness  of  field. 
The  relative  rapidity  of  these  series  \sf-4'$)f-6'3  and/- 8, 
but  the  first  and  second  are  not  now  listed.  Two  other 
lenses  were  patented  by  Dr.  Rudolph,  prior  to  the 
Protars.  Their  construction  is  shown  in  figs.  57  and  58. 


FIG.    57. 


FIG.    58. 


These  were  soon  abandoned  for  those  we  have  just 
described.  They  were  inferior  to  the  corresponding  series 
of  Aplanats  in  spherical  correction,  and  though  the  field 
was  flatter,  over  an  angle  of  about  50°,  there  was  greater 
astigmatic  difference. 

At  the  close  of  1891  Dr.  Rudolph  had  finished  the 
calculation  of  a  series  of  single  lenses  resembling  in 
construction  the  half  of  the  well-known  Goerz  Double 
Anastigmat,  for  which  Emil  von  Hoegh  made  an 


1 86 


ANAST1GMATIC  LENSES. 


FIG.  59. 


application  for  a  patent  in  December,  1892.  The  con- 
struction of  the  latter  is  shown  in  fig.  59.  Dr.  Rudolf 

Steinheil  had  also  been  en- 
gaged in  calculating  a  lens 
similar  to  the  Goerz,  but  as 
his  application  for  patent 
was  not  made  till  March  of 
the  following  year,  it  was 
too  late.  Dr.  Rudolph's  ap- 
plication for  a  German  patent; 
for  the  single  lens,  was  not 
made  till  a  month  later  than 
Dr.  Steinheil's,  and  it  was 

refused  for  the  same  reason.  Between  the  lenses  of 
Emil  von  Hoegh  and  Dr.  Rudolf  Steinheil,  on  the 
one  hand,  and  that  of  Dr.  Rudolph,  on  the  other 
hand,  there  is  a  characteristic  difference.  The  former 
made  the  independent  correction  of  the  two  halves 
subservient  to  the  correction  of  the  entire  objective,  but 
the  latter  aimed  at  obtaining  the  best  correction  of 
the  separate  combinations,  so  that  they  might  be  used 
to  best  advantage  alone,  and  yet  be  available  for  the 
construction  of  sets  of  doublets.  The  Goere  lens  was 
issued  in  two  series,  one  with  an  aperture  of  f-J"] 
''subsequently  increased  to/ 6'8),  and  another  with  an 
aperture  of/-i  I  for  copying  purposes. 

The  patent  application  of  Dr.  Rudolf  Steinheil,  to 
which  we  have  referred,  specified  two  types,  however, 
and  the  validity  of  the  claim  for  the  second  was  admitted 
after  considerable  delay.  The  construction  of  the 


ANASTIGMATIC  LENSES. 


187 


Second  Series,  now  known  as  the  Orthostigmat,  Type 
II.,  is  shown  in  fig  60.  Here  we  have  another  instance 
of  a  lens  construction  being  worked  out  independently 
by  two  opticians.  Dr.  Kampfer,  a  director  of  the  firm 
of  Voigtlander  &  Son,  of  Brunswick,  applied  for  a 
patent  for  a  lens  resembling  the  Orthostigmat,  Type  II. 


But  although  his.  application  was  refused,  he  ap- 
pears to  have  been  more  fortunate  in  the  treatment  he 
received  at  the  hands  of  Dr.  Rudolf  Steinheil  than 
the  latter  received  from  the  Charlottenburg  firm.  Dr. 
Steinheil  granted  Messrs.  Voigtlander  a  free  licence  to 
manufacture  Type  II.,  whereas  for  the  right  to  manu- 
facture Type  La  royalty  was  demanded  from  Dr.  Steinheil, 
but  declined.  The  Orthostigmat,  Type  II.,  is  issued  in 
four  rapidities:  f-6'8,  /-8,  /-io,  and  /-I2.  Messrs. 
Voigtlander's  lens,  the  Col  linear,  is  made  in  three  series, 
/-5'4,/-6'8,  and  /-io  to/-i2'5. 

In  1895  Dr.  Rudolph  calculated  another  series  of 
convertible  lenses  for  the  firm  of  Zeiss.  Each  com- 
bination consists  of  four  elements,  and  the  correction  is 
more  perfect  than  in  the  older  series  with  three  elements 


i88 


ANASTIGMATIC  LENSES. 


only.  The  construction  of  a  doublet  of  the  new  series 
is  shown  in  fig.  61.  These  lenses  are  known  as  Protars 
also,  the  single  combinations  being  called  Series  VII., 


FIG.   6l. 

and  the   doublets  Series  VI I  A.      The  construction  is  a 
further  development  of  Dr.  Rudolph's  idea  of  combining 
an  old  with  a  new  achromat.    By  referring 
to  fig.  62,  it  will   be   seen  that  the  front 
combination   has   been   turned    the    other 
way  and  cemented  to  the  back,  to  form 
a  compound  of  four  elements. 
It   is  possible   to    analyse    the 
triple  combination  similarly,  as 
shown   in   fig.  63,   by  dividing 
the   middle   element.     But  upon   examining 
the  glasses,  it  will  be  found  there  is  a  differ- 
ence     The  analysed  triple  combination  con- 
sists of  an  old  and  a  new  achromat,  in  which 
both  the  negative  elements  are  of  flint.     But 
in  Protar  VII. ,  the  new  achromat   is   formed   of  a  high 
refracting  crown,  cemented  to  a  lower  refracting  crown. 


FIG.  62. 


FIG.  63. 


AN  ASTIGMATIC  LENSES.  189 

Nevertheless,  the  gradation  of  the  refractive  indices 
maintains  the  same  order  as  in  the  simple  Protar. 
We  find  in  both  a  similar  arrangement  of  the  contacts, 
a  convex  and  a  concave  surface  being  turned  to  a 
medium  of  higher  refraction.  The  single  lenses  forming 
Series  VII.  have  an  aperture  of  f-n  in  the  small  sizes 
and  f-i 2*5  in  the  larger.  The  doublets  of  Series  VIlA. 
have  various  apertures,  viz.:  f-$'6  in  the  small  sizes, 
/-6'3  in  the  larger,  and/-/  and/"-/*/  in  those  combining 
lenses  of  different  foci.  Two  similar  single  lenses  may 
be  used  for  stereoscopic  purposes,  or  as  a  doublet. 
Two  different  lenses  give  the  photographer  the  choice 
of  three  different  foci,  and  three  different  lenses  the 
choice  of  six  foci.  This  is  a  great  convenience,  but  as 
the  lenses  are  each  composed  of  four  elements,  they  are 
necessarily  expensive. 

Shortly   after    the    introduction    of  the   convertible 
Protars,  Messrs.  Goerz  and  Von  Hoegh  also  applied  for 
a  patent  for  lenses  of  the  convertible  kind.     The  single 
lenses,  like  the  Zeiss  Series  VII.,  give  fine 
definition  at  full   aperture.     They  are  also 
corrected  for  astigmatism   and  flatness  of 
field.     Fig.  64  represents  a  single  lens  of 
this   type.     It  consists   of  three   negative,        FIG.  64. 
enclosing  two  positive,  elements.     Starting 
at    the    diaphragm,   we    find    near    it    a   biconcave    of 
lowest    refraction,    connected    to    a    biconvex    lens    of 
high   refraction.      Astigmatism   is   cured   by   their  con- 
tact,  and    as   the  curve   should    have  as  long  a  radius 
as    possible   to   flatten   the   field,   the    two   glasses   are 


190  ANA  STIC  MA  TIC  LENSES. 

selected  for  great  difference  of  refractive  power.  The 
middle,  or  third  lens,  is  biconcave  and  of  low  refraction. 
The  light  is  collected  by  the  contact  with  the  second 
lens,  and  it  exercises  a  compensating  influence  upon  the 
distortion  produced  by  the  other  surfaces,  without 
adding  to  the  astigmatism.  The  fourth  lens  is  biconvex, 
and  its  contact  with  the  fifth  is  used  for  correcting 
spherical  aberration.  The  choice  of  the  glasses  for  these 
two  lenses  is  governed  by  two  considerations.  Firstly, 
the  last  surface,  through  which  the  light  passes  before 
reaching  the  plate,  should  refract  as  much  as  possible, 
therefore  the  index  of  the  glass  for  the  fifth  lens  should 
be  high.  Secondly,  the  curve  of  contact  of  the  fourth  and 
fifth  lenses  must  be  of  definite  depth,  otherwise  the  dis- 
tortion would  be  in  excessofthecorrectivepowerofthecon- 
tact  of  the  second  and  third  lenses.  A  glass  of  given 
lower  refractive  power  must  therefore  be  selected  for  the 
fourth  lens.  A  consideration  of  the  following  indices  of 
refraction,  taken  from  the  English  Patent  Specification, 
will  throw  further  light  upon  the  subject  :-  — 1*51  ;  r6i  ; 
1*52;  1*54  and  i'6i.  It  will  also  be  noticed  that  the  posi- 
tive elements  are  of  smaller  diameter  than  the  negative, 
and  it  is  therefore  necessary  to  cement  a  positive  to  a 
negative  before  the  second  surface  of  the  positive  can  be 
finished.  The  negative  elements  are  thus  cemented 
together  at  the  margins,  and  as  they  are  truly  centred, 
the  positive  elements  must  also  be  true  in  this  respect. 
This  feature  is  the  essential  condition  of  the  German 
patent,  and  it  is  unique  as  the  means  of  protecting  by 
patent  the  construction  of  a  lens.  The  single  lenses 


ANASTIGMAT1C  LENSES. 


191 


work  at  full  aperture, /-I  I,  and  maybe  combined  to 
form  symmetricals  working  at  /-5*5,  or  convertible 
doublets  varying  in  rapidity  fromy-5'9  toy-63. 

Another  patent  was  obtained  by  Dr.  Rudolph  in 
1897,  for  a  lens  known  as  the  Planar.  In  it  we  find  an 
application  of  the  Gauss  method  of  correcting  the 
telescope.  Alvan  Clark,  an  American  optician,  made 
an  attempt  to  use  this  method  of  correction  for  photo- 
graphic lenses  in  1889,  Dut  without  marked  success. 
The  construction  of  the  Planar  is  shown  in  fig.  65.  The 


FIG.  65. 

cemented  pairs  are  of  crown  and  flint,  with  the  same  or 
approximately  the  same  refraction,  but  different  dis- 
persion. This  compound  lens  has  the  same  refractive 
power  for  a  given  colour  as  a  similar  one  made  of  either 
kind  of  glass,  but  the  dispersion  is  different.  Moreover, 
the  dispersion  may  be  modified  by  altering  the  contact 
curve.  The  va'ue  of  the  Gauss  method  of  correction 
lies  in  the  elimination,  for  a  large  aperture,  of  the 
error  known  as  the  chromatic  difference  of  spherical 
aberration.  As  the  cemented  lens,  to  which  we  have 


1 92  ANASTIGMATIC  LENSES. 

referred,  gives  the  optician  greater  latitude  in  the 
selection  of  the  glass,  the  Planar  construction  is  very 
accommodating.  The  quality  of  the  definition  of  these 
lenses  is  such,  that  they  may  be  used  as  low-power 
micro^:cpic  objectives.  In  the  smaller  sizes  the  intensity 
is/-3*6,  which  renders  them  very  useful  for  photomicro- 
graphy. The  field  is  exceptionally  flat  and  free  from 
astigmatism.  An  apochromatic  series  for  three-colour 
work  is  also  supplied. 

In  the  same  year  application  was  made  by  Dr. 
Rudolph  for  a  patent  for  the  lens  known  as  the  Unar, 
shown  in  fig.  66.  Both  its  com- 
binations are  formed  by  two  lenses 
enclosing  an  air  space.  The  focus 
of  the  front  combination  is  nega- 
tive, and  that  of  the  back  positive. 
In  this  respect  it  differs  from  the 

FIG.  66. 

Planar,  and  other  doublets  with  air 

lenses,  which  till  then  had  been,  without  exception,  of 
symmetrical  construction,  or  approximately  so.  The 
development  of  this  lens  and  that  of  the  Antiplanet, 
constructed  by  Dr.  Adolf  Steinheil,  is  suggestive  of  a 
parallel.  The  approximately  symmetrical  construction 
of  the  Planar,  an  objective  with  air  spaces  resembling 
one  another  in  their  effect,  is  abandoned  for  one  with  air 
spaces  of  opposite  character,  and  the  front  lens  becomes 
negative  in  focus.  The  parallel  is  not  complete,  yet  a 
striking  resemblance  exists  between  the  line  of  thought 
which  underlies  both  constructions.  In  the  patent  specifi- 
cation Dr.  Rudolph  gives  the  following  explanation  : — 


ANASTIGMATIC  LENSES.  193 

'  The  effect  of  combining  two  pairs  of  facing  sur- 
faces '  (those  enclosing  the  air  spaces)  '  of  opposite 
power,  is  similar  to  the  result  obtained  in  the  objective 
described  in  Specification  No.  6028,  A.D.  1890 '(fig.  55, 
p.  184),  '  by  the  opposite  sign  prescribed  by  the  difference 
between  the  refractive  indices  of  the  crown  and  flint 
lenses  in  the  cemented  components  of  a  doublet.  The 
pairs  of  facing  surfaces  produce,  in  accordance  with  the 
signs  of  their  powers,  astigmatic  differences  of  opposite 
character,  so  that,  in  addition  to  spherical  correction 
of  the  whole  system  and  flattening  of  the  image,  astig- 
matism may  be  fully  corrected.'  After  pointing  out 
that  the  use  of  air  spaces  introduces  a  greater  difference 
between  the  media  through  which  the  light  has  to  pass, 
and  increases  the  number  of  elements  available  for 
correcting  the  objective,  Dr.  Rudolph  adds  : — '  From 
the  foregoing  it  will  be  understood  that  the  adoption 
of  the  new  type  of  objective  will  result  either  in  larger 
apertures,  the  spherical  correction  remaining  of  the 
same  quality,  or — when  the  apertures  are  unaltered — in 
improving  spherical  corrections.'  Two  series  of  these 
lenses  have  been  issued :  the  aperture 
of  the  first  ranges  from/~-4'5  to/*- 5  "6. 
according  to  length  of  focus  ;  that  of 
the  second  is/-6'3  f°r  a^  tne  sizes. 

In  1902  Dr.  Rudolph  obtained 
a  patent    for   the    '  Tessar.'      The 
construction  of  this  lens  is  shown 
in  fig.  67.     The  front  combination  resembles  that  of  the 
Unar.     It  has  an  air  space  between  the  lenses,  and  is 

O 


t94  ANASTIGMATIC  L&NS&S. 

of  negative  focus.  The  back  combination  is  a  new 
achromat  of  positive  focus,  and  resembles  that  of  the 
Protar.  It  may  thus  be  looked  upon  as  a  cross 
between  these k  two  prior  types.  The  ordinary  series 
has  an  aperture  of  f-6"$.  There  is  also  an  apochro- 
matic  series  for  copying  and  three-colour  work,  the 
aperture  of  which  ranges  from  f-io  to  f-i$,  according 
to  focal  length.  This  lens  is  at  present  the  last  of  the 
photographic  lenses  invented  by  Dr.  Paul  Rudolph. 

In  1900  a  patent  was  granted  to  C.  P.  Goerz  for  a 
lens  which  is  of  great  value  under  certain  exceptional 
conditions,  where  an  abnormally  wide  angle  has  to  be 
included  in  the  photograph.  Otherwise,  the  droll  effects 
of  perspective  it  produces  might 
entitle  the  lens  to  rank  as  a  freak. 
Its  construction  is  shown  in  fig.  68, 
and  it  is  called  the  Hypergon 
Double  Anastigmat.  It  is  un- 
corrected  for  colour  and  spherical 
Flr  68  aberration,  but  it  possesses  an 

anastigmatic  flat  field,  and  covers 

the  exceptionally  wide  angle  of  about  135°.  The  aperture 
is  small,  being  only  /-22.  The  components  are  very 
thin,  and  their  surfaces  are  struck  as  nearly  as  possible 
with  the  same  radius  to  cure  astigmatism.  The  com- 
ponents are  nearly  hemispherical,  to  permit  the  in- 
clusion of  the  very  wide  angle  referred  to.  The 
unequal  illumination  of  the  objective  is  compensated 
by  a  revolving  starstop,  which  cuts  off  light  from  centre 
to  margin  in  diminishing  degree.  The  intensity  is  con- 


ANASTIGMATIC   LENSES.  195 

sequently  lower  than  the  aperture  of  the  stop  indicates. 
On  the  other  hand,  there  should  be  little  absorption  ot 
light  by  the  glass. 

The  last  series  of  lenses  brought  out  by  C.  P.  Goerz 
forms  the  subject  of  a  British  patent  granted  to  him 
and  Emil  von  Hoegh  in  1898.  From 
fig.  69,  which  shows  the  construction, 
it  will  be  seen  that  it  resembles  the 
Orthostigmat,  but  with  the  exception 
that  an  air  space  takes  the  place  of 
the  middle  element  in  both  combina- 
tions. This  meniscus,  in  the  Orthos- 

FIG.  69. 

tigmat,  has  the  lowest  refractive  index, 
but  as  air  is  of  still  lower  refractive  power,  the  difference 
in  the  gradation  of  the  indices,  upon  which  the  correc- 
tion depends,  is  considerably  increased.  This  has  been 
used  to  give  the  objective  a  larger  aperture.  There  is  a 
rapid  series  with  relative  apertures  ranging  from/^4'5  t° 
f-S'S)  and  a  slower,  with  the  relative  aperture  f-6'3  for 
each  size. 

Dr.  Rudolf  Steinheil  applied  in  1901  for  a  British 
patent  for  a  new  lens,  which  has  been  called  the 
'  Unofocal.'  Fig.  70  illustrates  the  construction,  which 
is  symmetrical.  This  lens  also  exhibits  a  resemblance 
to  the  Orthostigmat,  but  there  is  a  remarkable  differ- 
ence. The  refractive  indices  of  the  crown  and  flint 
are  equal.  The  achromatism  is  adjusted  by  a  definite 
distance  of  separation  between  the  elements,  and  the 
Petzval  rule,  requiring  that  the  sum  of  the  foci  shall 
equal  o,  is  complied  with.  In  the  Goerz  lens  previously 


196 


ANASTIGMATIC  LENSES. 


referred  to,  as  in  the  Orthostigmat,  the  biconvex  front 
lens  is  a  highly  refracting  crown  belonging  to  the 
series  of  anomalous  glasses  introduced  by  Schott  and 
Genossen.  In  a  cemented  lens  this  is  necessary  for 


FIG.    70. 


curing  astigmatism  by  the  combination  of  a  normal 
with  an  abnormal  pair  of  glasses.  But  in  the  Unofocal 
both  glasses  are  of  the  same  refractive  index,  and 
thus  we  have  neither  an  abnormal  nor  a  normal  pair. 
Yet  if  we  turn  to  Dr.  Rudolf  Steinheil's  statement  of 
the  conditions  necessary  to  cure  astigmatism  and 
spherical  aberration,  we  find  that  they  are  satisfied. 
Air  being  a  medium  of  lower  refraction  than  glass,  the 
space  between  the  two  elements  presents  the  requisite 
concave  and  convex  surfaces  to  media  of  higher  re- 
fraction. The  series  already  introduced  have  the 
apertures  f-4'$  and  f-6. 

Through  Dr.  Miethe's  acceptance  of  a  professorship 
at  the  Berlin  Technical  School,  a  vacancy  occurred  in 
the  directorate  of  the  firm  of  Voigtlander  &  Son.  Dr. 
Harting,  at  that  time  a  member  of  the  staff  of  the 
firm  of  Carl  Zeiss,  was  appointed  to  fill  Dr.  Miethe's 


AhASTIGMATIC  LENSES. 


197 


former  position.  In  December,  1900,  Dr.  Harting 
made  application  for  a  British  patent  for  the  lens- 
construction  shown  in  fig.  71.  This  objective  is  a 
triplet  of  symmetrical  construction,  corrected  for  astig- 
matism and  spherical  aberration  at  large  apertures. 
The  scheme  of  correction  may  be  described  as  fol- 
lows : — The  crowns  of  the  external  combinations  must 


FIG.    71. 

have  a  larger  or  smaller  index  of  refraction  than  the 
flints  to  which  they  are  cemented.  The  middle  lens  must 
be  of  opposite  kind  of  glass  to  either  element  facing 
kt,  the  refractive  index  of  one  being  smaller,  or  approxi- 
mately as  large,  and  the  dispersion  greater  than  that  of 
the  other.  In  the  drawing  the  outer  negative  lenses 
are  of  flint,  and  the  positive  lenses,  to  which  they  are 
cemented,  of  crown  possessing  higher  refraction  and 
lower  dispersion.  The  middle  equi-concave  lens  must 
therefore  be  of  flint,  possessing,  in  comparison  with  the 
crown,  the  same  or  lower  refraction  combined  with 
higher  dispersion.  Eighteen  months  later  Dr.  Harting 


198  ANAST1GMATIC  LENSES. 

applied  for  another  patent  embodying  improvements, 
in  the  objective  we  have  just  described.  The  difference 
in  the  construction  may  be  seen  by  comparing  fig.  72 
with  the  previous  one.  The  deviations  do  not  refer  to 
symmetry  of  arrangement  and  relative  proportionate 
sizes  of  the  indices  of  refraction  and  dispersion,  but  to 


FIG.  72. 

the  curves  of  the  lenses  and  the  choice  of  the  indices  of 
the  glass.  The  greater  freedom  thus  obtained  permits 
of  much  more  effective  correction  of  astigmatism  and 
curvature  of  field,  but  rectilinearity  is  affected  slightly, 
and  likewise  the  achromatism  of  the  focal  lengths. 
Under  these  patents  the  *  Heliar,'  with  an  aperture  of 
/"-4'5,  has  been  brought  out.  In  its  construction  it  re- 
sembles fig.  72. 

More  recently  Messrs.  Voigtlander  have  introduced 
the  *  Dynar,'  the  other  alternative  construction  of  the 
patents.  In  this  case,  the  position  of  crown  and 
flint  in  the  outer  combinations  has  been  reversed.  It 
follows  from  the  patent  specification,  under  these  cir- 
cumstances, that  the  central  negative  lens  must  be  of 


ANASTIGMATIC  LENSES. 


199 


FIG    73. 


highly  refractive  crown.  The  aperture  of  the  Dynar  is 
f-6.  Fig.  73  shows  its  construction. 
In  November,  1900,  a  photo- 
graphic objective  corrected  on  the 
Gauss  principle  was  patented  in  this 
country  by  Hugo  Meyer,  of  Gorlitz. 
The  construction  is  illustrated  in 
fig.  74.  The  claim  provides  that 
either  or  both  elements  shall  have 
one  convex  and  one  concave  surface.  The  positive 
element  must  be  of  high  refraction  and  low  dispersion, 
and  the  negative  of  similar  or  lower 
refraction  and  greater  dispersion 
than  the  positive.  The  crown  is 
therefore  one  of  the  Jena  new 
glasses  used  to  form  an  abnormal 
pair.  The  lens  is  also  patented  in 
Germany,  and  known  there  as  the  Aristostigmat.  The 
aperture  is/-77. 

Karl  Martin,  of  Rathenow,  also  applied  for  a  patent 
for  a  somewhat  similar  lens  in  September,  1901,  as  may 
be  seen  from  fig.  75,  which  shows 
its  construction.  In  it  a  negative 
meniscus  is  combined  with  a  posi- 
tive lens  to  form  an  air  space  be- 
tween them,  which  has  the  shape  of 
a  negative  lens.  The  negative  glass 
element  must  be  of  higher  refraction  than  the  positive, 
consequently  the  pair  of  glasses  is  a  normal  one.  The 
lens  is  corrected  for  colour,  astigmatism,  and  spherical 


FIG.  75. 


200 


ANASTIGMATIC  LENSES. 


aberration.       It    is    manufactured    by   the    Rathenower 

Optische    Industrie    Anstalt    in    two    series,    with    the 

relative  apertures  /-$'$  and  f-7'7- 

In  January,  1899,  Ernst  Leitz,  of  Wetzlar,  obtained 

a  British  patent  for  the  lens  construction  shown  in 
fig.  76,  called  the  Periplan.  The 
front  combination  resembles  in  con- 
struction the  original  Goerz  Anas- 
tigmat,  whilst  the  back  combination 
is  a  new  achromat.  The  whole 
burden  of  spherical  correction  is 


FIG.  76. 


thrown  upon  the  contact  of  the  meniscus  and  biconcave 
elements  of  the  front  combination.  The  other  two  con- 
tacts are  used  for  correcting  astigmatism.  The  relative 
aperture  of  this  objective  is/7'8. 

In  July  of  the  same  year  a  German  patent  was 
granted  to  the  same  optician  for  a  photographic  objec- 
tive of  symmetrical  character,  which 
has  been  named  the  '  Summar.' 
The  construction  is  exhibited  in 
fig.  77.  The  biconvex  crown  of 
high  refraction  is  u^c^j^for  correct- 
ing astigmatism.  '  The  -  plano- 
concave flint  cemented  to  the 
meniscus  of  low  refracting  crown 
form  together  a  negative  lens,  and  the  enclosed  contact 
corrects  the  spherical  aberration.  The  objective  has  a 
relative  aperture  of/- 5. 

A    German    patent    was    granted    to    E.    Arbeit    in 
February,    1901,  for   a   lens   constructed   as    in    fig.   78. 


FIG.  77 


ANASTIGMATIC  LENSES.  201 

The  objective  is  symmetrical,  both  combinations  being 
similarly  formed  of  a  new  achromat,  separated  by  an  air 
space  from  a  highly  refracting  crown  meniscus.    The  posi- 
tive elements  areof  the  same  kind 
of  glass.    The  lens,  the  Euryplan, 
is    made  by   Gebruder    Schulze, 
of  Potsdam,  who  have  introduced 
two     series     with     the     relative 
apertures  f-6  and  f-f$. 

The  preceding  anastigmatic  FIG.  78. 

lenses,    without    exception,    are 

of  German  origin,  but  the  list,  though  a  long  one, 
does  not  include  all.  Some  are  omitted  because  they 
have  not  reached  the  commercial  stage,  or  closely 
resemble  others,  which  have  been  included.  Yet  those 
which  are  described  make  it  apparent  how  powerful  has 
been  the  impulse  given  to  the  optician  by  the  intro- 
duction of  new  varieties  of  glass.  Even  in  England  two 
opticians,  Harold  Dennis  Taylor  and  Hugh  Lancelot 
Aldis,  have  made  noteworthy  efforts  to  improve  the 
photographic  lens.  The  work  of  the  former,  which  is  of 
great  originality,  dates  from  1893,  when  a  patent  was 
applied  for,  which  forms  the  basis  of  the  Cooke  lens 
constructions.  The  meniscus  form  of  the  positive  lens, 
used  in  the  construction  of  the  Aplanat  or  Rectilinear, 
is  definitely  abandoned,  because  it  involves  the  use  of 
diaphragm  corrections.  In  its  place  a  biconvex  is  used, 
and  the  radii  of  its  surfaces  are  given  certain  relative 
lengths  to  eliminate  coma.  The  negative  lens,  used 
for  achromatising  the  combination,  is  dealt  with  to 


202 


ANASTIGMATTC  LF.N<ES. 


eliminate  coma  of  opposite  character.  Its  focal  length 
approximates  to  that  of  the  positive,  a  condition  laid 
down  with  precision  by  Petzval.  One  of  its  functions 
is  to  increase  the  focus,  and  as  the  errors  of  curvature  of 
field  are  opposite  for  the  two  lenses,  they  tend  to 
produce  a  flattened  anastigmatic  image,  without  the  use 


FIG.  79. 

of  diaphragm  corrections.  Were  the  objective  con- 
structed of  two  elements  only  in  the  manner  described, 
the  image  would  be  distorted.  This  is  remedied  by 
dividing  the  positive  lens  and  placing  the  negative 
between  them,  thus  forming  a  triplet  Various  series 
have  been  issued.  Fig.  79  represents  a  portrait  combina- 
.  tion  of  f~4'$  relative  aperture. 

Fig.  80  represents  another  series 
with  apertures /-6*5  and /-8.  Ano- 
ther patent  was  applied  for  in  1899 
to  extend  the  usefulness  of  these 
FIG.  80  objectives  by  means  of  supple- 

mentary   lenses    formed    of    glass 

of  different    dispersive   powers.      These   supplementary 
lenses    vary   in    focal    length    approximately    as   their 


ANASTIGMATIC  LENSES 


203 


dispersion,  and  thus  considerable  variations  in  the 
focal  length  of  the  objective  may  be  secured.  Subse- 
quently another  patent  was  obtained  for  a  mechanical 
device  by  which  the  objective  may  be  focussed  by 
altering  the  distance  of  separation  between  the  front 
and  middle  lenses. 

In  September,  1895,  Mr.  Hugh  Lancelot  Aldis,  then 
with  the  firm  of  J.  H.  Dallmeyer,  Ltd.,  obtained  a  patent 
for  an  improvement  in  photographic  lenses.  These  con- 
structions have  been  named  the  Stigmatic,  and  three 
series  have  been  introduced.  Fig.  81  represents  the 


FIG.  81. 

portrait  lens  with  a  relative  aperture  of  f-^.  The 
inventor  turned  to  the  method  of  correction  used  by 
Dr.  Adolf  Stcinheil  in  the  Antiplanet.  The  front 
combination  has  strong  positive  spherical  aberration, 
which  is  corrected  by  sufficient  negfltive  spherical  aber- 
ration in  the  back  combination.  All  the  positive  lenses 
are  of  heavy  barium  silicate  crown.  The  negative 
lens  in  the  front  combination  is  an  ordinary  light  flint, 


204 


ANASTIGMATIC  LENSES. 


FIG.  82. 


and  the  two  negative  elements  in  the  back  combination 
are  of  soft  silicate  crown.  The  contact  of  the  cemented 
middle  pair  corrects  the  astig- 
matism. Fig.  82  represents  the 
universal  series,  which  has  an 
aperture  of  f-6.  In  this  the 
cemented  elements  are  of  heavy 
barium  silicate  crown  and  ordi- 
nary light  flint.  The  detached 
negative  meniscus  is  of  soft 

silicate  crown.  The  astigmatism  in  this  series  is  cor- 
rected by  the  two  cemented  contacts.  Both  com- 
binations may  be  used  as  landscape  lenses,  the  front 
being  about  twice,  and  the  back  about  one  and  a 
half  times,  the  focal  length  of  the  entire  objective.  A 
very  convenient  range  of  foci  is  thus  obtained  in  a 
compact  form. 

After  leaving  Messrs.  J.  H.  Dallmeyer,  Ltd.,  this 
optician  applied  for  another  patent,  which  was  granted 
in  February,  1902.  The  con- 
struction is  known  as  the 
Aldis  lens  and  it  is  shown 
in  fig.  83.  Spherical  aber- 
ration is  corrected  by  the 
cemented  surfaces  of  the  front 
combination,  and  astigmatism 

is  cured  by  the  inner  surface  of  the  back  lens.  The 
leading  idea  of  the  construction  appears  to  have  been 
economy  of  means  of  correction.  The  number  of  ele- 
ments has  been  reduced  to  the  fewest  possible,  and  in 


ANASTIGMATIC  LENSES. 


205 


this  respect  it  presents  a  very  marked  difference  if  it 
be  compared  with  the  portrait  construction  by  the 
same  optician.  The  aperture  of  this  lens  is  /-6,  but 
another  series  has  been  introduced  with  an  aperture 

of/-77- 

Messrs.  Watson  &  Sons  have  also  brought  out  an 
anastigmatic  lens,  which  may  be  used  in  convertible 
sets.  Fig.  84  illustrates  the 
construction.  Two  combina- 
tions of  similar,  o_r  dissimilar, 
focus  may  be  united,  as  in 
the  case  of  the  Zeiss  Con- 
vertible Protars.  The  con- 
struction appears  to  be  similar 
to  that  of  one  alternative  re- 


FIG.  84. 


ferred   to  in   the  patent  granted    to    Emil  von   Hoegh 
in  1892.     This  lens  is  called  the  Holostigmat. 

It  may  be  pointed  out,  in  conclusion,  that  the 
anastigmats  of  German  origin  are  broadly  divisible 
into  two  groups.  One  is  constructed  of  old  and  new 
achromats,  with  cemented  surfaces,  and  the  other  is 
dependent  upon  the  use  of  air  spaces  in  the  combi- 
nations. The  fact  that  the  Steinheil  Unofocal,  the  Karl 
Martin,  and  Cooke  lenses  may  be  made  without  ano- 
malous pairs  of  glasses  shows  that  an  achromatic  ob- 
jective, corrected  for  spherical  aberration  and  anastig- 
matic flatness  of  field,  might  have  been  constructed 
before  the  introduction  of  the  new  Jena  glasses. 
Perhaps  the  prejudice  against  air  spaces  in  the  con- 
struction of  a  lens  may  have  deterred  opticians  from 


2o6  ANAST1GMAT1C  LENSES. 

experimenting  in  this  direction.  Two  other  novelties  also 
deserve  special  notice  —  the  use  of  the  biconvex  and 
biconcave  elements,  approximately  free  from  diaphragm 
corrections,  introduced  by  Mr.  H.  Dennis  Taylor,  and 
the  hyperchromatic  dispersive  lens  used  by  Dr.  Rudolph 
in  the  Planar. 


CHAPTER  XXXI. 

MOUNTS     AND     CELLS. 

MANY  years  ago  a  French  optician,  Derogy,  intro- 
duced a  lens,  now  disused,  but  the  mechanical  features 
of  which  deserve  more  general  recognition  than  appears 
to  have  been  accorded  to  it.  It  was  formed  in  several 
different  sizes  (three  at  any  rate),  these  being  the  quarter, 
half,  and  whole-plate  portrait  lenses.  In  the  normal  con- 
dition these  were  lenses  of  good  quality,  suited  for  the 
different  dimensions  of  plates. 

Bayonet  Joints  for  Lens  Cells  and  Fittings. — Upon  dis- 
section and  examination  of  these  objectives  certain 
peculiarities  become  apparent.  First  of  all,  the  cells 
containing  the  lenses  are  not  adapted  to  the  mount  by 
screws,  but  by  means  of  '  bayonet  joints,'  there  being 
two  such  fastenings  fitted  to  each  cell.  The  workman- 
ship being  good  there  is  no  chance  of  anything  becoming 
unfastened.  On  removing  the  front  cell,  which  is  done 
by  a  quarter  of  a  turn  of  the  hand,  it  is  found  to  contain 
the  means  for  adapting  still  another  cell,  the  position  of 
which  will  be  nearly  midway  between  the  front  and  back 
lenses.  The  object  of  this  third  cell-receptacle  is  seen 
when,  upon  opening  a  small  circular  morocco  case,  which 


io8  FOCtfS. 

is  packed  in  the  bond  of  the  lens,  two  cells,  each  con- 
taining a  supplementary  lens  —  one  concave  and  the 
other  convex,  both  being  achromatic  —  are  disclosed 
neatly  fitted  in  appropriate  receptacles.  Either  of  these 
can,  with  a  quarter  turn  of  the  hand,  as  before,  be  trans- 
ferred to  the  vacant  place  in  the  mount,  and  thus  serve 
to  modify  the  focus. 

Lengthening  or  Shortening  the  Focus. — The  real  effi- 
ciency of  the  system  will  be  seen  from  the  following 
measurements  which  we  have  made  of  the  equivalent 
foci  of  the  one  such  lens  in  our  possession  when  sub- 
jected seriatim  to  its  several  modifying  influences  : — 
Premising  that  the  lens  now  being  described  is  one 
of  the  smallest  which  were  made,  namely,  the  quarter- 
plate  size,  and  that  the  diameter  of  the  front  and  back 
elements  is  slightly  under  one  inch  and  three-quarters, 
in  the  combined  form  as  a  double  portrait-lens  the 
equivalent  focus  is  seven  inches.  The  insertion  of  the 
cell  containing  the  concave  achromatic,  and  upon  which 
is  engraved  '  Lentille  pour  faire  plus  grandl  lengthens 
the  equivalent  focus  to  nine  inches  ;  while  the  sub- 
stitution for  it  of  that  containing  the  convex  achromatic, 
and  which  bears  the  inscription  '  Lentille  pour  faire  plus 
petit,'  shortens  the  focus  to  five  and  a  quarter  inches 
equivalent,  or  three  and  a  half  inches  back,  focus.  But 
the  front  lens  is  also  adapted  for  being  used  alone,  for 
which  purpose  it  is  transferred  to  the  place  of  the  back 
combination,  previously  removed  from  its  position,  giving 
a  focus  of  eleven  inches.  This,  however,  is  not  all,  for 
by  employing  the  front  and  the  concave  together  a 


DISTANCE  BETWEEN  LENSES.  209 

focus  of  seventeen  inches  is  obtained — the  substitution 
of  the  convex  for  the  concave  in  this  relation  giving  a 
focus  of  eight  inches. 

Here,  then,  are  great  capabilities  condensed  in  a 
small  space.  In  this  one  objective  we  have  foci  to  the 
following  extent : — Five  and  a  quarter  inches,  seven 
inches,  eight  inches,  nine  inches,  eleven  inches,  and 
seventeen  inches.  We  have  an  idea  that  this  com- 
bination has  long  ceased  to  be  manufactured ;  but  it 
is  probable  that  the  causes  which  led  to  its  having 
fallen  into  desuetude  are  now  removed,  and  we  describe 
it  as  containing  merits  to  which  manufacturers  might 
well  pay  heed.  Incidentally  we  may  state  that  one  of 
the  combinations  formed  is  that  which,  after  many  years' 
experiment,  has  been  found  by  Professor  Woodward  to 
be  best  adapted  for  use  with  his  solar  camera  as  an  ob- 
jective. The  combination  alluded  to  is  that  in  which 
the  convex  supplementary  lens — an  achromatic  meniscus 
—is  utilised  for  the  purpose  of  shortening  the  focus  of 
the  portrait  objective. 

Distance  between  Lenses  should  not  be  Arbitrary.  —  It 
does  not  follow  that  in  the  case  of  a  combination  lens 
the  distance  at  which  they  are  set  apart  in  the  mount  is 
the  best  for  every  purpose.  The  optician  has  to  make  a 
compromise,  and  secure  a  balance  of  advantages.  That 
distance  at  which  flatness  of  field  is  best  attained  may 
be  attended  with  flare,  while  an  increased  angle  of  view 
may,  under  certain  circumstances,  be  secured  without 
any  serious  loss  by  setting  the  lenses  much  closer  to- 
gether. The  most  generally  useful  mount  for  a  lens  of 


210       DISTANCE  OF  STOP  IN  SINGLE  LENSES. 

this  class  is  one  in  which  each  lens  is  set  in  a  short 
supplementary  tube,  capable  of  being  drawn  out  from 
the  common  centre,  so  as  to  increase  or  shorten  the  dis- 
tance between  them  at  will.  When  the  lenses  are  sepa- 
rated to  the  maximum  extent,  the  field  will  be  flat  even 
to  the  verge  of  astigmatism  with  a  large  aperture ;  while, 
in  proportion  as  they  are  made  to  approach  each  other, 
so  does  the  area  of  illumination  increase,  this,  how- 
ever, being  attended  with  roundness  of  field.  Hence, 
by  adopting  suitable  precautions  in  the  separation,  a 
doublet  lens  may  be  made  to  act  either  as  a  wide  or 
narrow  angle  objective.  The  expediency  of  adopting  a 
mount  of  this  kind  is,  however,  open  to  question,  as 
there  might  not  be  one  out  of  ten  who  would  know 
how  to  use  the  power  aright  were  it  placed  in  their 
hands. 

Distance  of  Stop  in  Single  Lenses. — A  very  sensible 
advantage  may  frequently  be  derived  by  the  power  of 
adjusting  the  distance  between  the  stop  and  the  lens  in 
the  case  of  a  single  landscape  objective.  It  is  well  known 
that  with  all  such  lenses,  especially  those  of  a  plano- 
convex or  only  slight  meniscus  form,  the  farther  the 
stop  is  from  the  surface  of  the  lens,  the  wider  may  be 
the  aperture  in  such  stop.  This,  however,  circumscribes 
the  field  of  delineation.  By  placing  the  stop  nearer  to 
the  lens,  two  advantages  are  secured.  First,  the  lens  will 
cover  a  much  larger  plate,  and,  secondly,  the  distortion 
that  is  so  common  to  landscape  lenses  becomes  mini- 
mised ;  for,  as  we  have  shown  in  a  previous  chapter,  the 
nearer  the  stop  is  to  the  optical  centre  of  a  lens,  the  less 


AL  UMINIUM  MO  UNTS.  2 1 1 

is  the  distortion  :  but  this  approximating  of  the  dia- 
phragm to  the  lens  necessitates  a  smaller  stop  being 
employed  than  when  a  greater  distance  intervenes  be- 
tween them. 

Cell-bound  Lenses. — It  is  of  vital  consequence  that  a 
lens  be  not  set  in  its  cell  under  conditions  which  give 
great  pressure  to  any  part  of  its  substance.  A  delicate, 
well-constructed  lens  may  have  its  good  qualities  dis 
turbed  by  being  forced  into  a  tight  cell  which  is  bur- 
nished down  upon  it,  thereby  giving  considerable  pressure. 
The  presence  of  this  pressure  is  readily  ascertained  by 
placing  the  lens  in  a  beam  of  polarised  light,  and  ex- 
amining it  by  an  analyser,  by  which  the  strain  on  the 
glass  will  be  shown.  The  effect  of  this  is  precisely 
as  though  the  lens  had  been  made  of  badly  annealed 
glass. 

Aluminium  Mounts. — The  weight  of  the  brasswork  of 
lenses  is  often  far  in  excess  of  what  is  required  for 
rigidity.  By  adopting  papier  mache,  ebonite,  or  alu- 
minium, an  important  saving  to  the  wear  and  tear 
experienced  by  the  photographer  would  be  effected.  It 
was  at  one  time  objected  to  aluminium  that  it  was 
expensive.  This  was  true  to  some  extent,  although  not 
so  much  so  as  to  render  its  applicability  to  a  photo- 
graphic lens  of  great  importance  in  this  respect.  But  it 
is  now  the  case  that  owing  to  the  demand  which  has 
arisen  for  this  metal,  its  price  has  been  reduced  to  that 
at  which  copper  is  now  sold.  As  the  specific  gravity 
of  aluminium  is  about  2*56,  while  that  of  copper  is  fre- 
quently 8*96,  the  great  gain  in  lightness  will  be  apparent. 


212  FLANGE  APERTURES. 

Some  makers,  notably  in  America,  have  begun  to  discard 
brass  for  the  diaphragms  of  their  larger  lenses,  adopting 
ebonite  or  vulcanite  instead,  to  the  great  advantage  of 
the  users.  It  only  remains  that  this  measure  of  reform 
shall  be  made  to  permeate  the  other  portions  of  the 
mount  to  have  an  improvement  far  exceeding  that  which 
was  inaugurated  by  the  introduction  of  the  leather  cap 
in  lieu  of  the  heavy  brass  cap  which  it  supplanted. 

By  the  apparent  paradox  of  making  use  of  heavy- 
glass  the  opticians  are  now  able  to  give  us  lenses  small 
in  bulk  and  comparatively  light  in  weight,  so  far  as 
concerns  the  mere  glasswork  of  the  objective.  It  now 
devolves  upon  them  to  effect  a  similar  measure  of 
reform  in  the  mounts  of  the  larger  of  the  portable  form, 
such  as  those  exceeding  one  and  a  half  inches  in 
diameter. 

Dimensions  of  Flange  Apertures. — It  is  much  to  be 
regretted  that  up  to  the  present  time  no  really 
universal  system  of  diameters  of  apertures  and 
screw  threads  in  lens  flanges  has  yet  been  adopted, 
notwithstanding  the  efforts  of  committees  of  Con- 
gresses, Conventions,  and  Societies  to  bring  about  so 
desirable  an  end.  A  practical  outcome  of  the  chaos 
that  still  prevails  is,  that  three  or  four  lenses  may  be 
purchased  from  as  many  different  leading  opticians,  and 
although  the  screws  on  these  might  have  so  easily  been  . 
absolutely  as  they  are  nearly  identical,  not  one  of  them 
will  interchange  with  the  others  in  the  flange.  This 
for  many  years  has  been  a  sore  grievance  with  users 
of  lenses. 


LENS  ADAPTERS.  213 

A  Universal  Lens  Adapter.— Pending  the  adoption  of 
some  system  on  which  all  makers  will  agree,  we  give 
here  a  method,  originated  in  France,  by  which  lenses 
having  various  flange  apertures  can  be  quickly  adapted 
in  succession  to  any  camera. 

A  series  of  discs  of  ebonite  or  thin  metal,  one  for 
each  lens,  is  provided.  They  are  all  of  equal  diameter 
outside,  but  the  aperture  in  each  is  such  as  just  to 
allow  the  screw  of  the  mount  to  pass  easily  through. 
The  flange,  which  is  smaller  than  the  disc,  is  now  screwed 
on  the  mount  and  keeps  the  disc  firmly  fixed.  The  hole 
in  the  camera  front  is  smaller  than  the  disc,  which,  when 
placed  over  it,  entirely  covers  the  aperture.  Three  guide 
pins,  or,  by  preference,  a  round  recess  in  the  camera- 
front,  ensures  the  lens  being  centrally  attached,  and  a 
turn-button  at  each  side  secures  it  firmly  to  the  front. 

This  method  is  equally  useful  for  the  studio  as  for 
the  field  camera.  It  permits  of  one  lens  being  changed 
for  another  in  a  very  brief  period  of  time,  and  saves  the 
trouble  of  having  a  separate  camera  front  for  each  lens 
that  is  likely  to  be  used. 

Lens  adapters  constructed  on  the  iris  diaphragm 
system  is  another  French  idea.  They  grip  a  lens  with 
a  closeness  sufficient  to  prevent  the  admission  of  light, 
but  the  hold  taken  of  the  lens  by  the  thin  edges  of  the 
iris  blades  is  rather  too  slender  to  ensure  the  lens  against 
dropping  out  at  an  inopportune  moment 


CHAPTER    XXXlJt. 

LENS  GRINDING. 

ALTHOUGH,  as  we  have  stated  in  the  Preface,  this 
work  is  intended  for  users  and  not  manufacturers  of 
lenses,  yet  may  there  be  some  among  the  former  who 
desire  to  know  how  lenses  are  ground  and  finished. 

Selection  of  the  Glass. — As  all  dealers  in  optical  glass 
supply  it  of  the  requisite  degrees  of  refractive  and  dis- 
persive indices,  no  trouble  now  arises  in  procuring  it. 
But  having  been  obtained,  it  is  necessary  to  subject  it  to 
careful  examination  for  internal  defects  which  would 
otherwise  only  be  discoverable  after  the  labour  of  grind-- 
ing and  finishing  the  surfaces  had  been  undergone,  and 
the  labour  thus  wasted. 

Imperfect  annealing  demands  primary  attention. 
This  defect,  where  it  exists,  is  readily  discoverable  by 
examining  the  glass  by  polarised  light.  Let  A  (Fig.  85) 
be  a  lamp,  C  ten  plates  of  clean  glass  bound  together 
at  the  edges,  and  E  a  Nicol  prism.  The  light  from  A 
becomes  polarised  when  reflected  from  C  at  a  suitable 
angle;  and  when  any  object,  D — in  this  case  the  slab  of 
glass  undergoing  examination — is  placed  in  the  path  of 
the  reflected  ray,  any  heterogeneousness  in  the  glass 


GRINDING.  215 

arising  from  imperfect  annealing  is  rendered  plainly 
visible  by  rotating  the  analyser,  E.  Incidentally  we  may 
observe  that  by  this  means  defects  in  finished  lenses  can 
also  be  discovered.  For  example,  an  otherwise  perfect 


FIG.  85. 

lens,  if  subjected  to  undue  pressure  in  its  cell,  will  show 
lines  or  patches  of  opacity  when  subjected  to  this  test. 

To  effect  the  conversion  of  a  piece  of  plain  glass 
into  a  lens  is  a  class  of  mechanical  work  demanding  no 
exceptional  degree  of  skill,  although  care  is  necessitated. 
Where  genius  is  required  is  in  the  determination  of 
the  curves  to  suit  the  special  requirement  and  of  the 
glass  best  adapted  to  the  purpose.  An  able  mathe-  [ 
matician  can,  as  the  result  of  his  calculations,  send  to  l 
the  manager  of  a  lens-grinding  establishment  a  formula 
or  specification  for  a  lens  as  to  which  he  can  predicate 
before  the  work  is  commenced  everything  as  regards 
its  capabilities  and  performance.  This,  however,  belongs 
to  the  'fine-art'  department  of  the  business  and  to  the 
higher  mathematics.  We  must  here  confine  ourselves  to 
the  more  material  aspects  of  the  construction  of  a  lens. 


216        RULES  FOR  FINDING  PRINCIPAL  FOCUS. 

Density  influences  Curvatures. — The  density  of  the  glass 
determines  the  curvature  requisite  in  making  a  lens  of 
a  definite  focus;  but  the  following  rules  and  expla- 
nations will  serve  to  afford  an  average  or  general  idea 
of  the  relation  between  focus  and  curvature.  On  the 
supposition  that  we  are  dealing  with  crown  glass  :  if  a 
circle  be  made  on  a  sheet  of  paper  with  any  opening  of 
the  compasses — say  three  inches — and  a  portion  of  this 
circle  be  cut  off  by  a  straight  line,  such  portion  will 
represent  a  plano-convex  lens  of  three  inches  radius, 
and  its  focus  for  parallel  rays  will  (assuming  the  convex 
surface  to  be  directed  outwards)  be  nearly  upon  the  line 
of  the  circle  opposite  to  the  lenticular  slice.  This  is 
more  tersely  expressed  in  treatises  on  mathematical 
optics  as  follows : 

Eule  for  Finding  the  Principal  Focus  of  a  Piano -Convex 
Lens. — When  the  convex  side  is  exposed  to  parallel  rays 
the  focal  distance  will  be  equal  to  twice  the  radius  of 
its  convex  surface,  diminished  by  two-thirds  of  the 
thickness  of  the  lens. 

Rule  for  Finding  the  Principal  Focus  of  an  Equally 
Double -Convex  Lens. — The  focal  distance  is  equal  to  the 
radius.  In  the  drawing  which  we  have  imagined  above 
if,  instead  of  the  portion  of  the  circle  having  been 
separated  by  a  straight  line,  a  curved  line  of  the  same 
radius  as  the  circle  had  been  employed,  the  lens 
formed  would  have  come  under  this  category,  namely, 
equally  double-convex,  and  its  focus  would  have  been 
approximately  in  the  centre  of  the  circle,  or  three 
inches. 


GRINDING  TOOLS.  ^1 

Rule  for  Finding  the  Focus  of  a  Double-Convex  Lens  of 
Unequal  Curvatures. — Multiply  the  radius  of  one  surface 
by  the  radius  of  the  other,  and  divide  twice  this  product 
by  the  sum  of  the  same  radii.  This  last  lens  is  usually 
designated  a  '  crossed '  lens. 

In  the  case  of  a  meniscus  with  parallel  rays  we  must 
divide  twice  the  product  of  the  two  radii  by  their 
difference,  and  the  quotient  will  be  the  focal  distance 
required. 

These  rules  must  not  be  considered  absolute,  for 
with  every  different  sample  of  glass  there  may  be  a 
departure  from  them,  and,  in  some  cases — e.g.,  dense 
flint — the  departure  will  be  very  considerable  ;  but  with 
ordinary  crown  or  plate  glass  they  are,  probably,  as 
near  as  can  be  framed  in  popular  language. 

Grinding  Tools. — Having  determined  upon  thediameter 
and  curvature  of  the  lens  to  be  made,  the  first  thing  to 
do  is  to  obtain  grinding  tools  of  the  radius  of  curvative 
required.  They  consist  of  a  pair — namely,  a  convex 
and  a  concave— and  can  be  purchased  of  any  radius 
from  those  who  make  a  speciality  of  this  department  of 
business  ;  but  an  amateur  will,  doubtless,  prefer  to  make 
them  f6r  himself.  To  do  this  he  must  make  two  tem- 
plates of  thin  sheet  brass  or  zinc  (Fig.  86),  by  turning  one 
piece  to  exactly  three  inches  in  diameter,  assuming  that 
a  radius  of  three  inches  is  to  be  employed  ;  the  other 
piece  to  have  a  hole  of  this  diameter  cut  in  it,  and  after- 
wards divided  into  two  pieces.  To  make  a  concave 
grinding  tool :  provide  a  thick  and  substantial  piece  of 
brass  or  gun-metal  in  the  form  of  a  chuck,  and  with  a 


TOOLS. 


suitable  turning-tool  hollow  out  the  end  so  as  to  fit  the 
curvature  of  the  round  template.  This  may  necesskatc 
several  trials  if  the  amateur  be  inexperienced  in  the  use 
of  lathe  tools.  A  second  piece  of  brass  is  now  turned  in 
the  same  way,  but  so  as  to  be  the  exact  counterpart  of 


FIG.  86. 

the  preceding ;  that  is,  its  outer  end  must  be  rounded, 
and  this  curve  must  be  gauged  by  the  hollow  template. 
Both  tools  having  been  finished  by  the 
lathe  tool  as  well  as  possible,  they  are 
next  ground  one  upon  the  other  by 
friction,  with  the  interposition  of  a  little 
fine  flour  emery  and  water  until  they  fit 
each  other  with  great  nicety. 

To  prevent  waste  and  save  labour, 
glass-makers  now  supply  the  material 
moulded  appropriately  to  the  form  the 
lens  ultimately  assumes  ;  but  where  the 
raw  material  is  in  the  form  of  a  flat  slab, 
it  is  cut  into  squares,  each  of  which  is 
' shanked'  to  a  circular  form  by  means  FIG.  87. 
of  a  pair  of  shanks,  as  shown  in  Fig.  87,  these  being 


&&1NDTNG  TOOLS.  219 

made  of  soft  iron,  and  procurable  from   all  dealers  in 
opticians'  requirements. 

The  glass  having  been  nibbled  or  '  shanked '  to  a 
round  form  is  cemented  by  pitch  or  sealing-wax  to  a 
suitable  handle,  and  is  rough  ground,  either  on  a  grind- 
stone or  in  an  iron  mould  with  coarse  sand,  until  it  is 

nearly  the  shape  required. 
For  small  lenses  this  may 
be  effected  in  the  turning 
lathe  with  a  sharp  steel 
cutting  tool,  which  must 
be  kept  constantly  wet 
with  spirit  of  turpentine,  benzoline,  or  one  or  other  of 
several  liquids  of  a  similar  kind,  which  have  been  found 
to  answer  the  purpose  equally  well.  It  is  desirable  that 
a  flat  piece  of  glass,  B,  be  interposed  between  the  handle 
A  and  the  lens  C  to  prevent  marginal  errors  in  grinding. 
In  either  this  or  the  rough-grinding  method  the  tem- 
plate must  be  occasionally  applied  as  a  means  of  ascer- 
taining progress.  It  being  desirable  to  save  the  brass 
tools  as  much  as  possible,  the  more  effectively  the  first 


FIG.  88. 


I 


FIG.  89. 

grinding  is  done  in  the  coarser  tool  the  better  will  it  be 
for  the  chances  of  the  finishing  tool  preserving  its  form 
unimpaired  for  a  long  period.  Fig.  89  shows  a  convex 


220  GRINDING  THE  SURFACES. 

and  a  concave  tool  ready  for  insertion  in  the  turning 
lathe. 

Grinding  the  Surfaces. — Grinding  proper  is  effected  by 
means  of  emery,  of  which  several  grades  arc  employed 
In  large  cities  opticians'  emery  is  a  commercial  article. 
Those  who  prefer  to  make  it  for  themselves  may  do  so 
by  taking  a  quantity  of  flour  emery — say  a  pound — and 
placing  it  in  a  clean  jar.  To  this  add  water  and  stir  it 
about  until  it  is  all  wet  and  of  a  pasty  consistence. 
Now  add  water  to  fill  up  the  jar,  stir  the  whole  contents 
well  round,  and,  after  waiting  for  a  little  till  the  heavier 
particles  subside,  pour  off  the  water,  in  which  is  mixed 
up  the  lighter  portions,  into  a  second  jar,  which  fill  up 
with  water  and  stir  vigorously  as  before,  pouring  off  the 
water,  after  five  minutes,  into  a  third  jar.  This  is 
repeated,  a  longer  time  for  settling  in  each  case  being 
given.  The  result  of  this  washing  process  is  that 
while  in  the  first  jar  the  deposit  consists  of  the  coarsest 
portions  of  the  emery,  the  deposit  becomes  finer  and 
finer  as  the  washing  is  allowed  to  proceed,  till  at  last  the 
water  holds  in  suspension  only  the  very  smallest  atoms 
of  the  emery,  which,  when  precipitated,  forms  the  finest 
emery  capable  of  being  procured. 

To  smooth  the  roughly  ground  surface  of  the  lens  the 
coarsest  of  these  deposits  of  emery  is  first  employed, 
mixed,  of  course,  with  water.  When  upon  examination 
with  a  magnifier  the  surface  is  homogeneous,  the  grind- 
ing is  repeated  with  a  finer,  succeeded  by  a  still  finer, 
grade  of  emery,  until  at  last  the  convex  surface  of  the 
glass  is  so  fine  as  to  present  the  appearance  of  being 


POLISHING  THE  LENS.  221 

ready  to  burst  into  a  black  gloss.  At  this  stage  the 
operation  with  the  emery  terminates.  It  need  scarcely 
be  said  that  in  the  grinding  with  the  various  grades  of 
emery  careful  washing  must  be  resorted  to  between  each, 
and  that  the  grinding  with  any  one  class  of  emery  must 
be  continued  until  every  mark  made  by  its  predecessor 
has  been  removed.  Also,  in  course  of  the  grinding  it  is 
well  that  the  counterpart  of  the  tool  be  applied,  with  a  little 
emery  and  water,  so  as  to  ensure  its  being  kept  in  shape. 
Polishing  the  Lens. — To  impart  a  final  polish  we  have 
seen  several  methods  adopted.  One,  and  the  most 
primitive,  is  to  cement  on  the  face  of  the  grinding  tool  a 
piece  of  textile  fabric  of  a  fine  nature  from  which  the 
nap  has  been  removed  by  a  hot  iron.  Some  employ 
woollen  cloth,  others  fine  linen,  and  in  some  instances 
paper.  It  is  cemented  on  the  face  of  the  tool  by  pitch 
or  other  cement,  the  counter  tool  being  employed  to 
preserve  the  curve.  Rouge,  a  mixture  of  rouge  and 
putty  powder,  or,  not  unfrequently,  putty  powder  alone 
moistened  with  water,  is  employed  to  give  the  final 
polish.  When  the  finest  surface  possible  to  be  obtained 
is  desired,  instead  of  polishing  upon  cloth  or  linen  the 
tool  is  faced  with  pitch.  This  is  applied  by  warming 
the  tool  and  then  rubbing  over  it  a  piece  of  pitch,  which 
melts  and  coats  the  surface  in  a  uniform  manner.  It  is 
spread  more  evenly  by  the  application  of  the  counterpart 
tool.  A  little  rouge  or  putty  powder  is  spread  over  the 
surface  and  moistened  with  water.  On  applying  the 
surface  of  the  lens  to  this  with  rapid  friction  it  imme- 
diately receives  a  fine  black  polish. 


222  BLOCKING  LENSES. 

Putty  Powder. — The  best  way  to  make  putty  powder 
for  this  purpose  is  to  dissolve  tin  in  aqua  regia  and 
precipitate  by  diluted  ammonia.  Wash  the  peroxide  in 
several  changes  of  water,  and,  after  drying,  expose  in  a 
crucible  to  a  low  white  heat,  by  which  the  particles 
acquire  the  property  of  polishing  quicker  and  better. 
Owing  to  the  white  colour  of  the  putty  powder  many 
prefer  to  mix  with  it  a  little  rouge  or  crocus — not  alone 
to  modify  its  polishing  properties,  but  also  to  enable 
it  to  be  seen  when  on  the  cloth.  The  polishing 
powder  must  not  be  too  wet,  but  sufficiently  so  to 
take  a  partially  glazed  appearance  from  the  action  of 
polishing. 

Edging  and  Centering. — The  edging  of  the  lens  is 
effected  by  cementing  it  upon  a  chuck,  and  while 
rotating  in  the  lathe  the  reflection  of  the  flame  of  a 
candle  is  observed.  If  it  remain  quite  steady  all  is  right ; 
but,  if  not,  it  must  be  shifted  slightly  before  the  cement 
hardens  until  it  do  so.  A  piece  of  copper  or  brass  well 
supplied  with  emery  and  water  is  then  applied  to  the 
lower  edge,  an  even  pressure  being  given  until  the  edge 
is  smooth  and  the  lens  quite  round. 

Blocking  Lenses. — When  lenses  are  not  large  and  are 
to  be  ground  to  shallow  curves,  a  considerable  number 
may  be  cemented  on  a  block  and  operated  upon 
simultaneously.  In  this  way  upwards  of  two  dozen 
may  be  ground  and  polished  in  the  same  time  that  one 
would  take.  For  grinding  the  commoner  class  of  lenses, 
such  as  spectacle  glasses,  machinery  is  employed  in 
connexion  with  the  block  system, 


SPECIMEN  LENS  CURVES.  223 

Specimen  Lens  Curves. — It  would  be  foreign  to  the 
object  of  a  work  like  this  to  give  formulae  by  which  the 
curves  of  lenses  formed  of  the  many  different  kinds  of 
glass  now  procurable  may  be  ascertained,  but  it  may 
not  be  out  of  place  to  give  the  curves  (supplied  through 
the  courtesy  of  the  present  head  of  the  firm),  of  a  fine 
specimen  of  the  No.  2  wide-angle  lens  of  Dallmeyer,  of 
the  form  shown  on  page  44,  Fig.  18,  made  for  us  in 
1865.  Measuring  from  the  diaphragm,  the  radii  are — 

1.  -  5*253-  4-     +  4'3o6. 

2.  +    1*46.  5.       —  4*306. 

3.  —    1*46.  6.       +   2 '2. 

The  diameter  is  2  inches,  and  the  focus  8J  inches.  It 
is  made  of  Chance's  glass.  Soon  after  receiving  it  we 
found  that  it  would  bear  a  working  aperture  very  greatly 
in  excess  of  that  intended  by  the  optician,  and  for  over 
twenty  years  we  have  used  it  for  portraiture,  with  an 
opening  of  f-8.  When  stopped  down  it  covers  10  x  8 
easily. 


CHAPTER  XXXIII. 

OPTICAL  CONTACT — CEMENTING  LENSES. 

THE  fewer  the  reflections  in  or  connected  with  a 
lens  the  better,  because  the  invariable  tendency  of  these 
is  the  fogging  of  the  plate.  Some  lenses  distribute  the 
reflections  all  over  the  plate ;  in  the  case  of  others  a 
concentration  takes  place  upon  the  centre  of  the 
negative.  The  former  is  not  good,  and  the  latter  is 
highly  objectionable. 

What  we  here  mean  by  reflections  will  be  better 
explained  by  a  demonstration.  Take  a  portrait  lens 
and  step  with  it  into  a  darkened  room.  Light  a  candle 
and  place  it  at  a  distance  of  a  few  feet ;  then  hold  up 
the  lens  in  the  line  of  the  candle  light,  when  a  repeated 
duplication  of  the  image  of  the  flame  will  be  seen,  some 
of  these  images  being  erect,  others  inverted. 

Reflections  Reduced  by  Cementing. — Now,  seeing  that 
the  fewer  reflecting  surfaces  there  are  in  an  objective 
the  fewer  will  be  the  number  of  these  reflected  images, 
of  course,  it  follows  that  the  multiplicity  of  such  surfaces 
is  an  evil,  and  for  this  reason  opticians  have  sought  to 
make  the  inner  surfaces  of  achromatic  lenses  '  contact 
curves '  as  far  as  possible.  The  reason  for  this  is 


CEMENTING  B  Y  BALSAM.  22 «; 

obvious  :  if  these  inner  surfaces  be  concentric  as  regards 
curvature,  it  is  only  necessary  that  they  be  placed  in 
optical  contact  to  ensure  a  nearly  total  elimination  of  the 
reflections  that  would  inevitably  arise  were  the  contact 
between  them  merely  mechanical  instead  of  being 
optical.  To  secure  the  latter,  all  that  is  necessary  is 
to  interpose  between  the  two  concentric  surfaces  any 
clear  fluid — such  as  water,  oil,  or  varnish — when  the 
interior  surfaces  that  could  previously  be  seen  by  looking 
down  upon  them  immediately  disappear,  and  the  lens 
appears  to  be  formed  of  one  homogeneous  piece  of 
glass. 

Cementing  by  Balsam.  —  Of  the  various  substances 
employed  in  the  cementing  of  achromatic  lenses,  that 
which  is  most  generally  preferred  is  Canada  balsam  ; 
for  it  is  easy  of  application,  possesses  the  requisite 
degree  of  transparence^  and  dries  quite  hard.  There 
is  a  well-grounded  objection  to  the  employment  of  this 
substance  for  large  telescopic  object-glasses,  because 
the  expanding  ratio  of  flint  and  crown  glasses  being 
different,  they  will  be  affected  by  thermal  influences, 
which  would  cause  a  strain  owing  to  the  two  unequally 
expanding  bodies  being  securely  cemented  together. 
To  obviate  this  a  permanently  fluid  body — e.g.,  castor 
oil — is  recommended  in  preference  to  balsam  for  lenses 
of  this  class. 

The  photographer  who  wishes  for  ocular  demon- 
stration as  to  the  advantages  arising  from  cementing 
a  lens  can  obtain  it  in  the  following  manner : — Provide 
two  clean  pieces  of  glass,  such  as  quarter-plates,  and, 


226  ARBORESCENT  MARKINGS. 

holding  one  of  them  in  a  level  position,  allow  a  drop  of 
oil  to  fall  upon  it.  Now  lay  the  second  plate  on  the 
top  of  the  other  so  as  to  cover  and  flatten  out  the 
drop  of  oil.  Observe  how  transparent  the  glasses  have 
become  by  the  cementing  of  the  inner  surfaces  in 
the  manner  described.  Wherever  the  oil  touches  both 
surfaces  optical  contact  is  secured.  The  experiment 
just  described  serves  to  demonstrate  the  difference 
between  optical  and  mechanical  contact,  and  also  to 
show  the  brilliancy  arising  from  the  cementing  of  two 
surfaces  of  glass. 

Almost  without  exception  the  front  lens  of  the 
portrait  combination  and  both  lenses  of  the  '  rapid ' 
class  of  objectives  are  cemented;  but  the  cement  not 
unfrequently  undergoes  changes  and  vicissitudes  by 
which  the  performance  of  the  objective  is  seriously 
damaged.  We  shall  here  describe  the  nature  of  some 
of  these  changes  and  the  means  of  cure. 

Arborescent  Markings  in  Balsamed  Lenses.— Occa- 
sionally, after  a  portrait  combination  has  been  some 
time  in  use,  an  arborescent  growth,  commencing  with 
a  single,  delicate,  leaf-like  form,  appears  at  one  side 
of  the  front  lens,  and  gradually  spreads  inwards.  If 
the  balsam  has  been  very  thin  when  applied,  this 
arborescence  spreads  over  a  large  portion  of  the  sur- 
face. One  of  the  finest  examples  of  this  defect  occurred 
in  the  back  lens  of  one  of  our  10  x  8  'rapid7  objectives 
which  remained  good  for  about  four  years  after  being 
made,  and  then  had  a  beautiful  mass  of  shrubbery 
growing  all  round  the  margin.  This  increased  to  such 


DISCOLORATION  OF  TH'E  CEMENT.  227 

an  extent  as  to  leave  only  a  small  clear  spot — the  size 
of  a  threepenny  piece — in  the  centre.  This  is,  perhaps, 
the  most  prevalent  form  of  defect  in  the  cement  of  a 
lens. 

Discoloration  of  the  Cement. — Another,  which  also  makes 
its  appearance  after  the  lens  has  been  in  use  for  a  few 
years,  consists  in  a  discoloration  of  the  cement.  All 
round  the  margin  the  lens  is  found  to  have  become  of 
a  yellow  colour,  which,  although  at  first  pale,  afterwards 
becomes  more  decided,  and  not  unfrequently  assumes  a 
green  hue.  Eventually  the  lens  becomes  so  slow  in  its 
action  as  to  be  cast  aside,  and  to  have  its  place  supplied 
by  the  instrument  of  another  maker.  In  all  cases  of 
this  character  which  we  have  had  an  opportunity  of 
examining,  the  defect  in  question  invariably  arose  from 
the  lens  having  been  burnished  (or  screwed)  into  its  cell 
before  the  balsam  had  been  allowed  to  harden,  in  con- 
sequence of  which  an  action  had  set  up  between  the 
balsam  and  the  brass  cell  surrounding  the  lens,  resulting 
in  a  slow  decomposition  of  the  latter,  which  eventually 
coloured  the  balsam. 

There  are  some  kinds  of  balsam  which  acquire  a 
yellow  colour  through  age  ;  but  we  are  not  aware,  in 
our  own  experience,  of  any  thin  film — such  as  that 
which  forms  the  cementing  stratum  of  two  lenses  —  ever 
having  become  discoloured  by  light  to  an  extent  that 
could  be  appreciated.  On  the  contrary,  the  tendency 
of  light  is  to  bleach  it.  Time,  however,  and  exposure 
to  the  atmosphere  certainly  imparts  a  yellow  colour — 
a  fact  well  known  to  those  who  have  prepared  trans,- 


228       to  REMEDY  DEFECTIVE  CEMENTING. 

parent  paper  by  the  agency  of  Canada  balsam.  It  is 
also  known  to  microscopists  that  sometimes  slides  which 
have  been  prepared  with  balsam  have,  after  a  few  years, 
acquired  a  yellow  tint  somewhat  similar  to  that  which 
results  if  an  excess  of  heat  be  applied  in  the  preparation 
of  the  slide. 

To  Remedy  Defective  Cementing. — When  a  defect  in 
the  cementing  of  the  lens  is  observed,  or  when  a  dis- 
coloration is  suspected  owing  to  a  lens  working  more 
slowly  than  it  did  originally,  and  which  discoloration 
may  be  detected  by  laying  the  lens  upon  a  sheet  of 
white  paper  and  noting  its  appearance,  the  first  stage 
in  the  remedying  of  the  defect — supposing  the  photo- 
grapher elects  to  cure  it  himself  instead  of  sending  it 
to  an  optician — consists  in  removing  the  lens  from  its 
cell  into  which  it  is  fixed,  either  by  the  edge  of  the  cell 
being  turned  over  its  margin  or  by  a  screwed  ring. 

On  its  removal  from  the  cell,  the  lens  is  placed  in  a 
saucepan  on  the  bottom  of  which  is  laid  a  small  piece 
of  wood  to  prevent  the  contact  of  the  glass  with  the 
metallic  bottom.  Slightly  lukewarm  water  is  now 
poured  in  to  a  height  more  than  sufficient  to  cover 
the  lens,  and  heat  is  gently  applied  until  the  balsam 
has  become  so  soft  as  to  permit  the  lenses,  when  ma- 
nipulated by  the  fingers,  to  be  slidden  one  from  the 
top  of  the  other.  When  this  has  been  done,  the  water 
is  wiped  off  and  the  lenses  allowed  to  become  cold. 
Ether  or  collodion  is  now  poured  over  each  surface,  and 
gentle  friction  with  a  soft  cloth  applied.  By  this  means 
the  old  balsam  is  dissolved  and  entirely  removed.  Oil 


CEMENTING  THE  SURFACES.  229 

of  turpentine  or  benzole  answer  a  similar  purpose  as  a 
solvent.  The  cleaning  of  the  surfaces  is  finally  com- 
pleted by  means  of  soap  and  water. 

Some  have  recommended  the  use  of  the  carbonates 
of  potash  or  soda  as  a  solvent  for  the  balsam  ;  but  these 
are  bad,  on  account  of  their  action  on  the  glass. 

Cementing  the  Surfaces. — When  quite  clean,  and  wiped 
dry  by  means  of  wash-leather,  lay  the  flint  glass  on  a 
sheet  of  paper,  concave  side  up,  and  deftly  apply  a  large 
drop  of  the  finest  quality  of  Canada  balsam  to  the  centre, 
taking  care  that  it  is  free  from  air  bubbles.  Arrange- 
ments must  be  made  for  keeping  the  lens  quite  warm 
during  this  operation.  Now  lower  down  upon  it  the 
contact  surface  of  the  crown  glass,  and  by  gentle  pressure 
guide  it  so  as  to  cause  the  drop  of  balsam  to  expand 
equally  outwards  until  it  oozes  slightly  out  at  the  margin. 
Next  lift  it  up,  and  by  means  of  a  long  piece  of  soft 
string  tie  the  two  together,  crossing  and  recrossing  the 
string  in  every  direction.  This  ensures  their  being  kept 
in  a  central  position.  Heat  is  now  gently  applied  by 
laying  it  on  the  hot  plate  of  a  warm  but  not  superheated 
oven,  until  upon  removing  the  lens  and  testing  the 
balsam  which  has  oozed  out  at  the  edges  it  is  found 
to  be  hard.  Then,  having  allowed  the  lens  to  cool 
slowly,  remove  the  string,  and  clean  thoroughly  with 
ether  or  benzole.  The  lens  will  now  be  found  to  have 
become  rejuvenated. 


CHAPTER     XXXIV. 

SELECTION  OF  LENSES. 

Form  of  Lenses  for  Enlarging. — For  an  enlarging 
objective  with  the  solar  camera,  in  which  the  source  of 
light  partakes  more  of  the  nature  of  a  point  than  what 
we  have  been  considering,  the  construction  of  the 
objective  may  partake  of  a  far  wider  range  and  be  of  a 
more  diversified  character  than  any  of  the  others.  We 
have  seen  images  similar  in  dimensions  produced  from  a 
test  negative  in  which  the  objective  was  composed 
respectively  of  a  portrait  lens,  a  '  rapid  '  combination, 
and  an  achromatised  meniscus.  It  was  not  only  a 
difficult  matter  to  adjudicate  upon  the  respective  merits 
of  these  pictures,  but  experts  present  at  the  time  and 
having  before  them  examples  produced  by  each  system 
of  objective  were  found  to  have  arrived  at  varying 
conclusions  respecting  their  relative  merits.  In  conver- 
sation with  the  late  Dr.  van  Monckhoven,  who  had 
bestowed  much  attention  upon  the  subject,  that  gentle- 
man gave  it  as  his  opinion  that  the  best  of  all  objectives 
for  the  solar  camera  would  yet  prove  to  be  a  single 
achromatic.  Previous  to  that  time  he  had,  in  his  work 
on  Photographic  Optics,  in  1866,  in  the  portion  in  which 


LANDSCAPES.  231 

,  he  describes  his  enlarging  solar  camera,  spoken  of  its 
;  objective  as  having  the  '  external  form  of  Ramsden's 
•'  eye-pieces  placed  on  pocket  telescopes,  but  constructed 
on  the  principles  of  M.  Petzval's  doublet.'  That  the 
doctor  had  altered  his  opinion  subsequently  to  writing 
this  is  apparent  from  the  fact  that  to  none  of  the 
objectives  manufactured  by  him  at  a  later  period  does 
this  description  apply,  and  we  have  seen  several. 
Woodward,  of  Baltimore,  who  has  constructed  more 
solar  cameras  than  any  other,  makes  the  objectives  of 
best  '  solars '  of  three  achromatic  lenses,  the  front  and 
back  being  similar  to  those  of  the  ordinary  portrait  com- 
bination, but  having  the  focus  shortened  by  the  insertion 
of  a  third  meniscus  achromatic  lens  between  them. 

Enlarging  Portrait*. — If  the  subject  to  be  enlarged  is 
a  single  portrait,  say  of  carte  size  or  a  little  larger,  then 
will  a  carte  or  other  good  quarter-plate  portrait  lens  be 
found  to  be  the  most  suitable.  It  is  of  the  greatest  con- 
sequence, however,  that  the  back  lens  of  the  combination 
be  placed  next  to  the  negative,  otherwise  will  the  de- 
finition and  flatness  of  field  be  inferior.  There  will  be 
little  or  no  necessity  for  using  a  diaphragm  in  the  lens, 
as  the  area  of  sharpness  when  employing  full  apertures 
will  be  quite  sufficient  for  the  intended  purpose. 
[  Landscapes. — But  in  the  case  of  a  landscape  or  a 
group,  some  members  of  which  are  near  to  the  margin 
of  the  plate,  it  will  be  requisite  either  to  make  use  of 
a  diaphragm,  so  as  to  ensure  marginal  definition  of  the 
highest  class,  or  to  employ  a  lens  of  longer  focus.  The 
solar  focus  of  a  lens  is  not  its  focus  when  used  for 


232  LANDSCAPES. 

enlarging-,  more  especially  to  the  extent  of  only  a  few 
diameters,  and  hence  it  should  be  borne  in  mind  that 
the  focus  being  longer  when  thus  employed  its  covering 
power  is  extended.  A  combination  lens  of  the  '  rapid  ' 
doublet  type  will  be  found  excellent  in  the  case  of  a 
landscape,  in  which,  unlike  a  portrait,  the  marginal 
definition  must  equal  that  of  the  centre. 

If  time  of  exposure  be  of  no  consequence,  then  will  an 
achromatic  of  plano-convex  or  slight  meniscus  form 
answer  well  the  purpose  of  an  enlarging  lens.  But  it  is 
necessary  that  a  rather  small  top  be  employed,  that  it  be 
situated  at  not  less,  but  preferably  more,  than  the 
diameter  of  the  lens  from  its  flat  surface,  and  that  the 
convex  surface  of  the  lens  be  placed  next  to  the  nega- 
tive. There  will  be  a  residuum  of  distortion  when 
employing  such  a  lens,  but  in  the  case  of  a  landscape  or 
group  it  will  not  be  discoverable  in  the  large  picture 
which  results  from  the  operation,  and  this  being  so,  nice 
theoretical  considerations  concerning  rectilinearity  may 
be  placed  to  one  side.  But  if  any  curvature  of  a 
marginal  vertical  straight  line — as  in  the  case  of  a 
building — be  discoverable,  this  may  be  reduced  by 
removing  the  diaphragm  and  placing  it  closer  to  the 
lens.  This  applies  only  to  single  landscape  lenses,  and 
only  then  if  the  subject  be  an  architectural  one,  the 
vertical  lines  of  which  extend  to  the  margin  and  show 
indications  of  being  curved. 

In  an  objective  employed  in  enlarging  one  is  apt  to 
be  deceived  as  to  its  focus.  This  may  be  illustrated  by 
an  example.  Suppose  that  the  solar  focus  (equivalent) 


STEREOSCOPIC  LENSES.  233 

of  the  enlarging  objective  be  six  inches,  the  distance 
between  the  centre  of  the  lens  and  the  negative  to  be 
enlarged  would  be  six  inches  practically,  were  the  screen 
on  which  the  enlargement  is  projected  at  an  infinite 
distance.  These  two,  the  negative  and  the  screen, 
represent  the  anterior  and  posterior  conjugate  foci  ofthe 
lens.  But  as  such  a  position  ofthe  screen  is  impracti- 
cable it  must  be  brought  nearer,  and  as  there  is  a  strict 
relationship  between  the  conjugate  foci,  the  nearer  the 
screen  is  made  to  approach  the  objective  the  further 
must  the  negative  be  removed  from  it.  When  the  screen 
has  been  brought  so  near  as  to  show  the  image  of  the 
same  dimensions  as  the  negative,  then  if  a  careful 
measurement  be  made,  it  will  be  found  that  the  lens  has 
now  a  focus  of  twelve  inches,  or  double  that  it  possesses 
for  distant  objects.  The  anterior  focus  of  the  lens, 
represented  by  its  distance  from  the  screen,  is  now  found 
to  have  been  reduced  from  infinity  to  twelve  inches  also. 
Stereoscopic  Lenses.  —  For  securing  instantaneous 
stereoscopic  pictures  of  a  well-lighted  outdoor  scene  the 
great  majority  of  subjects  will  be  amenable  to  the  action 
of  a  single  lens  of  about  six  inches  focus.  This  admits 
of  the  employment  of  a  diaphragm  sufficiently  large  to 
permit  the  usual  class  of  subjects,  including  seaside 
groups,  boats,  &c.,  to  be  taken  in  a  quasi-instantaneous 
manner.  But  if  the  very  best  effects  as  regards  rapidity 
of  exposure  are  desired,  it  then  becomes  necessary  to 
employ  a  pair  of  portrait  combinations,  used  without  any 
diaphragm.  Of  these  the  finest  effects  will  probably  be 
obtained  with  a  back  focus  of  from  five  and  a  half  to  six 


234  LENSES  FOR  DIRECT  PORTRAITURE. 

and  a  half  inches,  as  this  gives  a  more  uniformly  lighted 
picture  than  when  an  objective  of  short  focus,  such  as 
three  and  three-quarters  inches  back  focus,  is  employed, 
as  was  frequently  the  case  in  those  days  when  instan- 
taneous stereoscopic  photography  was  prevalent. 

For  indoor  groups  and  scenes  it  is  probable  that  the 
regular  stereoscopic  portrait  combination  of  short  focus 
cannot  be  surpassed,  or  even  equalled,  for  general  utility. 
Its  small  diameter  and  short  focus  give  it  a  great 
penetrative  range,  while  its  large  *  angular  aperture ' 
enables  it  to  be  worked  with  great  rapidity.  Although 
on  this  account  we  advise  its  employment  in  preference 
to  any  other  in  a  room  in  which  it  is  desired  that  a 
scene  or  group  be  taken  with  a  short  exposure,  we  are 
strongly  of  opinion  that  for  outdoor  purposes  it  is  not  to 
be  commended,  unless  the  subject  to  be  taken  be  at  no 
considerable  distance  from  the  camera. 

Lenses  for  Direct  Portraiture. — For  children's  portraits 
it  is  necessary  that  the  lens  has  a  large  aperture,  seeing 
that  they  must  be  taken  with  the  briefest  of  exposures. 
To  this  end  a  Petzval  portrait  combination  will  form  the 
most  useful  lens,  this  being  employed  with  a  concealed 
pneumatic  shutter.  For  adults,  or  where  the  same 
rapidity  is  not  necessary,  either  a  portrait  lens  of  as  long 
a  focus  as  possible,  or  a  cemented  rapid  doublet  may  be 
used.  If  the  studio  is  badly  lighted,  the  former  will 
prove  the  more  useful.  If  a  large  head  and  bust  be 
wanted,  and  the  light  permit,  a  single  landscape  lens, 
working  with  a  large  aperture,  will  give  soft  and 
harmonious  pictures. 


COPYING  PORTRAITS.  235 

Lenses  for  Pure  Landscape. — Ordinary  landscapes  in 
which  architectural  subjects  do  not  form  a  chief  feature 
are  best  taken  with  a  landscape  lens — that  is,  a  single 
achromatic.  This  class  gives  bold,  crisp  definition,  this 
brilliancy  of  the  image  being  due  to  the  simplicity  of  the 
form  ;  for,  as  we  have  shown  in  a  previous  chapter,  the 
presence  of  a  second  lens  in  a  photographic  objective 
causes  the  formation  of  flare,  which,  when  not  confined 
to  one  central  spot,  becomes  diffused  over  the  negative, 
thus  leading  to  a  want  of  vigour  in  the  shadows.  This 
class  of  lenses,  when  employed  for  pictures  of  medium 
dimensions  in  which  the  included  subject  is  of  a  some- 
what small  angle,  is  capable  of  being  used  with  a  stop 
sufficiently  large  to  permit  good  negatives  being  obtained 
with  an  exposure  of  a  fractional  part  of  a  second. 

Groups. — When  a  group  is  to  be  taken,  either  in  a 
studio  or  in  a  dull  light  out  of  doors,  one  of  the  *  rapid  ' 
class  of  lens  with  an  intensity  of  from  f-'j  to/- 13, 
according  to  circumstances,  will  be  found  to  be  the  most 
useful.  This  lens  also  forms  the  best  objective  for  large 
portraits  in  the  studio,  which  it  produces  of  more  harmo- 
nious quality  than  a  large  portrait  lens  could  possibly  do. 

Copying  Portraits. — In  the  copying  of  a  portrait  it  is 
probable  that  photographers  will  invariably  use  the 
portrait  lens  they  commonly  employ.  This  class  of 
work  falls  quite  within  its  scope ;  but,  as  the  majority  of 
operators  first  focus  the  picture  and  then  insert  a  small 
stop  to  work  with,  we  caution  them  that  several  other- 
wise good  and  useful  portrait  lenses,  as  well  as  some 
specially  constructed  for  copying,  have  their  focus  altered 


236  ARCHITECTURE. 

by  the  insertion  of  a  smaller  stop  to  work  with  than  that 
by  which  the  focussing  was  effected.  This  is  not  always 
the  case ;  but,  as  it  is  sometimes  so,  it  is  a  wise  pre- 
caution to  use  the  full  aperture  of  the  lens  for  making 
the  general  arrangements  and  having  the  focussing 
effected,  and  then,  after  inserting  the  working  stop,  to 
take  a  final  look  at  the  image  on  the  ground  glass  and 
ascertain  the  state  of  its  sharpness  by  means  of  a  magni- 
fying-glass,  observing  whether  by  slightly  turning  the 
pinion  the  definition  is  not  capable  of  being  improved. 
Observe  this :  that  when  the  copy  is  required  to  be 
larger,  or  on  a  larger  scale,  than  the  original  it  is 
necessary  that  the  lens  be  turned  '  end  for  end '  so  as  to 
have  its  back  lens  nearest  to  the  picture  to  be  copied. 

Maps  or  Charts. — When  the  subject  to  be  copied  is  a 
map  or  chart  it  is  absolutely  necessary  that  a  non- 
distorting  objective  be  employed.  A  lens  of  the  'rapid' 
class  is  most  advantageous  for  this  kind  of  work. 

Large  Micro  Objects. — If  the  class  of  work  required  to 
be  reproduced  on  a  scale  of  magnification  be  flies  or 
insects  of  moderately  large  dimensions,  a  quick-acting 
locket  lens  will  answer  the  purpose  better  than  a 
properly  constructed  miscroscopic  objective  of  the  same 
focus,  as  f.he  former  has  its  chemical  and  visual  foci 
coincident  whereas  the  latter  has  not. 

Architecture. — Architectural  work  can  be  produced 
equally  with  *  rapid '  as  with  wide-angle  lenses,  provided 
these  be  of  a  non-distorting  class.  Distortion,  as  here 
meant,  implies  the  curving  of  lines  near  the  margin  of 
the  picture  which  are  straight  in  the  original. 


CHAPTER  XXXV. 

ON  THE  CURE  OF  EXISTING  DISTORTION. 

Distortion  of  Curvature. — A  single  landscape  lens,  more 
especially  when  made  to  take  in  a  wide  angle  of  view, 
gives  to  all  straight  lines  near  the  margin  of  the  view  an 
offensive  curvature  to  the  image,  which  becomes  in- 
creasingly great  as  these  recede  from  the  centre.  In 
ordinary  landscapes  this  is  quite  immaterial,  unless  when 
such  happen  to  be  bounded  at  either  side  by  a  very  tall 
building  which  extends  considerably  up  the  margin  of 
the  plate  ;  in  portraits  or  groups  the  distortion  of  curva- 
ture is  also  not  of  a  nature  that  can  be  discovered.  But 
quite  different  is  it  when  the  subject  of  the  photograph 
is  architecture  or  a  map  or  plan.  For  these  a  non- 
distorting  compound  lens  should  be  used,  but  we  are 
now  dealing  with  the  fact  that  such  has  not  been 
employed.  Here,  then,  we  are  confronted  with  a  nega- 
tive in  which  what  should  be  straight  lines  towards  the 
margins  are  bent  like  the  sides  of  a  barrel,  and  the 
question  is,  How  to  cure  it  ? 

The  first  thing  to  be  done  is  to  make  from  the 
distorted  negative  a  transparency  by  superposition  on 
the  same  sized  plate.  If  care  be  taken  to  have  negative? 


238          THE  DISTORTION  OF  CONVERGENCE. 

and  plate  in  perfect  contact  throughout,  and,  moreover, 
if  the  light  by  which  the  impression  is  to  be  made  be 
made  to  fall  upon  the  printing  frame  from  one  direction 
only,  there  will  not  be  any  loss  of  sharpness.  A  full 
exposure,  with  proper  development,  will  ensure  every 
detail  in  the  one  to  be  seen  in  the  other.  Too  great 
intensity  or  contrast  between  the  lights  and  shadows  is 
to  be  avoided.  We  have  now  got  a  transparency  as 
sharp  as  the  negative,  having  all  its  detail  and  also  all 
its  distortion. 

Now,  by  means  of  the  camera,  and  with  a  single 
lens,  preferably  of  shorter  focus  than  that  originally 
employed,  having  its  stop  next  to  the  sensitive  plate, 
make  a  negative  from  the  transparency.  As  the  negative 
thus  obtained  is  distorted  as  regards  the  exact  reproduc- 
tion of  the  transparency,  and  as  that  distortion  is  of  the 
opposite  character,  the  result  will  be  that  the  lines  which 
were  curved  in  the  original  negative  are  straight  in  the 
reproduced  one.  To  ensure  sharpness  the  diaphragm 
must  be  a  very  small  one,  any  remaining  traces  of  cur- 
vature, should  such  be  perceived  on  the  ground  glass, 
being  removed  by  placing  the  stop  nearer  to  or 
farther  from  the  lens,  for  the  more  the  margin  of 
the  lens  is  brought  into  requisition  the  greater  is 
its  power  of  producing  or,  in  this  case,  correcting 
distortion. 

The  Distortion  of  Convergence. — Every  one  knows  that 
if  a  camera  be  pointed  upwards  at  a  building,  so  as  to 
get  its  upper  part  into  the  picture,  the  perpendiculars 
will  converge,  being  narrower  at  the  top  than  at  the 


THE  DISTORTION  O&  CONVERGENCE.  2^9 

bottom.  Every  one  also  knows  that  this  convergence 
of  the  perpendiculars  may  be  altogether  prevented  by 
swinging  the  back  of  the  camera  so  as  to  cause  the 
ground  glass  to  stand  in  a  vertical  position.  It  is  not 
here  a  question  of  this  or  that  lens,  for  no  lens  has  been, 
or  can  be,  made  by  which  such  convergence  will  not 
result  if  the  sensitive  plate  be  not  vertical.  All  the  best 
cameras  are  now  provided  with  a  swing-back,  but  in 
those  of  the  hand  or  detective  class  it  is  seldom,  if  ever, 
to  be  found. 

Let  us  suppose,  then,  that  we  have  got  a  negative  of 
a  tall  building,  in  which,  from  tilting  up  the  camera 
without  having  brought  the  ground  glass  or  sensitive 
plate  into  the  vertical  position,  the  sides  converge  and 
lean  towards  each  other.  How  is  this  to  be  cured,  or 
more  correctly,  how  is  another  negative  to  be  produced 
from  it  in  which  no  details  shall  be  lost,  but  in  which 
the  converging  building  shall  be  restored  to  its  original 
perpendicular  position  ? 

First  of  all,  make  from  it  a  transparency  by  super- 
position in  a  printing  frame,  as  before,  and,  having 
erected  this  transparency  in  front  of  a  plate  of  opal 
glass  (by  preference),  and,  by  means  of  a  camera  fitted 
with  a  short-focus,  non-distorting  lens,  and  a  swing-back, 
focus  as  sharply  as  possible  with  the  largest  aperture, 
and  swing  back  the  ground  glass  until  the  convergence 
of  the  building  is  seen  to  be  neutralised,  and  the  vertical 
lines  rendered  parallel.  Now  insert  the  smallest  stop, 
so  as  to  ensure  top  and  bottom  being  equally  sharp,  and 
expose.  The  negative  which  results  from  this  treatment 


240  CURVATURE  AND  CONVERGENCE. 

Vvill  be  rectilinear,  and  in  every  respect  perfect  so  far  as 
drawing  is  concerned. 

The  Distortion  of  Curvature  and  Convergence  Combined.— 
This  compound  distortion  is  one  of  an  intensely  offensive 
nature.  In  it  not  only  are  the  marginal  straight  lines 
curved,  but  they  also  converge.  It  is  produced  in  its 
most  perfect  degree  by  having  a  camera  without  a  swing- 
back  fitted  with  a  wide-angle,  single-landscape  lens, 
and  pointing  it  well  upwards,  to  take  in  the  upper  part 
of  an  architectural  subject.  But,  provided  it  only  be 
sharply  defined,  it  is  amenable  to  being  perfectly  cured 
equally  as  in  the  former  cases. 

The  treatment  is  precisely  the  same  as  in  the  case  of 
the  distortion  of  convergence,  subject  to  this  difference, 
that  the  lens  by  which  the  cure  is  to  be  effected  must 
not  be  rectilinear,  as  in  the  former  instance,  but  must 
be  single,  as  in  that  necessary  for  curing  the  distortion 
of  curvature.  The  swinging  of  the  camera  back  ensures 
the  converging  lines  being  rendered  vertical,  while  the 
counter  distortion  of  the  lens  equally  ensures  their  being 
made  quite  straight. 


CHAPTER  XXXVI. 

LANTERN   OPTICS— ENLARGING  AND  PROJECTING. 

IN  this  subject  are  embraced  the  radiant  or  light,  the 
condenser,  and  the  object-glass  or  projecting  lens. 

The  Light.— This,  for  enlarging,  must  be  small  in 
dimensions  and  intense  in  quality ;  the  former  in  order 
to  obtain  sharpness,  the  latter  to  obviate  the  necessity 
of  giving  a  protracted  exposure. 

Mineral  Oil  Lamps. — These,  when  well  selected,  are 
extremely  convenient.  It  is  probable  that  on  account 
of  its  smallness  an  Argand  flame  possesses  greater  ad- 
vantages than  any  other.  It  will  undoubtedly  serve  the 
purpose  equally  as  well  as  the  limelight,  provided  a 
diaphragm  be  interposed,  close  to  the  flame,  to  cut  off 
the  top  and  bottom,  as  magnitude  of  the  radiant  in  an 
optical  lantern  conduces  to  impaired  definition.  The 
theoretically  perfect  light  is  one  in  which  dimensions 
scarcely  find  a  place ;  but  its  attainment  being  imprac- 
ticable we  must  do  the  next  best  thing.  Something 
much  higher  is  aimed  at  in  the  optical  requirements  of 
enlarging  than  those  which  obtain  when  a  luminous 
image  is  thrown  upon  a  screen  for  the  illustration  of  a 
lecture  or  the  delectation  of  juveniles  at  an  evening 
gathering.  Parallax  in  the  flame  must  be  avoided  as 

R 


242  LIMELIGHT. 

much  as  possible,  else  will  definition  of  a  high  class  be 
sought  for  in  vain. 

The  Marcy  Lamp. — The  Marcy  lamp  is  also  a  good 
one.  By  the  Marcy  lamp  we  mean  all  those  in  which 
the  burner  consists  of  more  than  one  flat  wick  turned 
endwise  to  the  condenser.  Marcy,  of  Philadelphia,  used 
two,  others  three,  four,  and  five.  The  principle  is  the 
same.  Various  names  are  now  given  to  this  system  of 
lighting  according  as  it  finds  development  in  the  lamps 
of  the  numerous  manufacturers  by  whom  it  is  issued. 
For  projections  it  is  powerful,  and  has  superseded  the 
Argand  burner,  but  we  have  now  to  look  at  something 
else  than  mere  power.  The  fact  of  the  edges  of  the 
various  flames  which  are  contracted  being  axial  in  the 
optical  system  implies  a  fulfilment  of  the  condition  of 
sharpness  in  its  highest  form  axially ;  but  when  oblique 
incidences  are  considered  then  are  we  met  by  magnitude 
of  flame,  and  consequent  parallax.  Nevertheless,  when 
the  relative  positions  of  the  flames  each  to  the  other 
are  such  as  to  prevent  vertical  lines  of  varying  luminous- 
ness  being  apparent  in  the  centre,  enlargements  of  fairly 
good  sharpness  and  equal  illumination  throughout  may 
be  obtained  by  its  agency. 

Limelight. — One  or  other  forms  of  the  limelight  may 
be  employed  with  unvarying  success.  The  blow-through 
jet  is  the  safest  and  simplest  when  carburetted  hydrogen 
or  common  house  gas  is  used.  Where  this  gas  is  not  acces- 
sible then  will  the  flame  of  a  spirit  lamp  answer  quite  well. 
The  blowpipe  from  the  cylinder  or  bag  of  oxygen  play- 
ing on  this  flame  causes  it  to  impinge  upon  a  cylinder  of 


ALBO-CARBON.  243 

lime,  which,  becoming  incandescent  under  the  great  heat, 
emits  a  powerful  light.  The  most  intense  form  of  this  light 
is  when  the  hydrogen  and  oxygen,  both  under  high  pres- 
sure, are  brought  into  mixture  just  before  they  issue  from 
the  orifice  of  the  burner.  The  light,  when  the  gases  are 
properly  regulated,  is  not  large,  but  exceedingly  intense. 
Albo-Carbon. — A  flame  of  common  gas  enriched  by 
'  albo-carbon,'  or  any  other  suitable  hydro-carbon,  has  in 
our  hands  as  well  as  in  those  of  others  proved  to  be  a 
very  suitable  light  for  enlargements.  Its  best  form  is 
that  which  we  introduced  at  the  1887  Conference  of  the 
Camera  Club,  and  consists  of  two  fishtail  burners,  sepa- 
rated from  each  other  by  the  extent  of  an  inch,  both 
flames  having  their  flat  sides  towards  the  condenser, 
there  being  an  opaque  disc,  with  a  circular  aperture  in 
it  of  a  little  over  half  an  inch  in  diameter,  placed  as 
close  as  possible  up  against  the  foremost  flame  so  as  to 
reduce  its  effective  area.  The  position  of  this  aperture 
must  be  such  as  to  be  opposite  to  the  most  luminous 
part  of  the  flame.  The  second  flame  behind  the  anterior 
one  serves  to  confer  intensity,  and  is  of  great  utility ; 
but  nothing  seems  to  be  gained  by  a  third  burner.  The 
gas  flame,  when  thus  enriched  by  the  vapours  of  the  albo- 
carbon,  becomes  very  intense.  An  Argand  flame  from 
gas  thus  enriched  ought  to  yield  a  light  of  great  excel- 
lence, provided  it  has  a  smaller  flame  ascending  through 
its  centre,  and  that  provision  is  made  to  condense  it  by 
diminishing  its  diameter  either  by  a  brass  solar  cap  to 
cause  a  strong  air  current  to  impinge  upon  the  flame 
a  little  above  the  burner,  or  by  a  contraction  in  the  glass 


244 


THE  CONDENSER. 


chimney.  Whiteness  and  intensity  in  such  a  case  are 
increased  by  a  judicious  lengthening  of  the  chimney  to 
increase  the  draught.  The  area  of  the  flame  must,  how-- 
ever,  be  reduced  by  the  expedient  already  pointed  out. 

The  Condenser. — The  use  of  a  condenser  is  to  gather 
together  rays  from  the  lamp  which  would  otherwise  be 


Fig.  90. 


Fig.  91. 

lost,  and  bend  them  in  such  a  way  as  to  pass  through 
the   negative   and   on   towards   the   objective.      Let  a 


THE  DOUBLE  CONDENSER.  245 

(Fig.  90)  be  the  radiant,  b  the  negative,  and  c  the  ob- 
jective. Notice  that  while  the  rays  are  transmitted 
through  the  negative  only  those  that  are  nearly  central 
or  axial  reach  the  objective.  Observe  now  what  takes 
place  when  a  lens  intercepts  the  rays  ere  they  reach  the 
negative.  They  are  refracted,  and  instead  of  getting 
lost  as  before  (as  shown  by  the  diverging  dotted  lines), 
they  have  become  deflected  in  the  direction  of  the  objec- 
tive, and  were  the  eye  placed  at  the  diaphragm  of  this 
objective  it  would  see  every  part  of  the  negative  brightly 
illuminated.  One  condenser,  however,  does  not  answer 
properly  on  account  of  the  spherical  aberration  that 
arises  when  the  focus  is  short,  as  it  must  necessarily  be. 
The  condensers  most  commonly  employed  in  optical 
lanterns  consist  of  two  plano-convex  lenses  mounted 


Fig.  92. 

close  together,  the  convex  surfaces  towards  each  other, 
as  shown  in  Fig.  92,  in  which  L  is  the  light,  C  the  con- 
densers, and  A  the  apex  of  the  projected  luminous  cone. 
This  form  answers  fairly  well  when  the  flame  is  large, 
butj.  unless  made  with  a  long  focus,  it  will  not  perform 
in  a  satisfactory  manner  when  the  flame  or  radiant  is 


246 


THE  DOUBLE  CONDENSER. 


small.  A  better  form,  if  only  two  lenses  must  be  em- 
ployed, is  to  have  a  plano-convex,  or  a  lens  of  slightly 
meniscus  form  (this  being  according  to  the  nature 
of  the  glass  employed)  working  in  conjunction  with  a 
bi-convex  lens  (Fig.  93).  When  the  curvatures  are  such 
that  the  rays  after  transmission  through  the  former  of 
these  fall  upon  the  latter  in  a 
parallel  direction,  then  must  the 
curves  of  the  latter  not  be  of 
equal  radius,  but  such  as  to 
make  it  a  'crossed  lens,'  in 
which  the  radii  are  as  one  to 
six,  or  nearly  so,  this  being  well 
known  to  be  a  form  that  is 
fairly  conducive  to  the  reduc- 
tion of  spherical  aberration.  But 
even  here,  unless  the  focus  of 
this  combination  be  long,  per- 
fection of  illumination  is  un- 
attainable. Why,  then,  not  make  it  long?  For  this 
reason,  that  the  loss  of  light  would  be  too  serious.  It 
is  of  primary  importance  that  the  light  in  a  lantern, 
whether  for  projecting  a  picture  for  examination  or 
for  enlarging,  be  powerful.  How  is  this  to  be  done  ? 
We  answer  it  by  requesting  attention  to  the  preceding 
diagram,  in  which,  although  the  lenses  are  not  so 
correctly  figured  as  they  ought  to  be,  the  principle  is 
plainly  enough  shown.  In  this  diagram  an  angle  of 
illumination,  say  of  ninety  degrees,  might  be  obtained, 
but  the  condensers  are  unable  to  grasp  more  than  c  and  dt 


Fig.  93- 


DOUBLE  AND  TRIPLE  CONDENSERS.  247 

and  even  a  little  less  than  this,  the  large  volume  from 
a  to  c  and  from  b  to  d  being  left  outstanding.  Now 
when  it  is  considered  that  this  represents  a  loss  of  about 
one-half  of  the  light,  its  reclamation  is  evidently  worthy 
of  attempting.  By  intercepting  these  lost  rays  by  a 
plano-convex  or  a  meniscus  lens,  which  need  not  be  of 
so  great  a  diameter  as  the  others,  they  are  by  it  secured 
and  made  to  impinge.  The  deduction  from  this  is  that 
a  triple  condenser  is  better  than  a  double  one. 

The  best  form  of  a  double  condenser  is  that  shown 
in   Fig.  94),  which   consists   of  a  plano-convex   and   a 


Fig.  94- 

crossed  lens,  the  flat  side  of  the  former  being  next  to  the 
light.  It  does  not,  however,  include  such  a  wide  angle 
as  those  of  triple  form,  although  when  well  made  its 

spherical  aberration  is  but  small. 
Triple  C  ndensers.— One  of  this 
class  which  we  devised  many 
years  ago,  was  composed  of  three 
piano  -  convex  lenses  (Fig.  95), 
the  centre  one  of  which  was 

achromatised.  and  that  farthest 
Fig.  95- 


248 


ACHROMATIC  CONDENSERS. 


from  the  light  of  colourless  crown  of  a  high  refractive 
index.  This  gave  excellent  illumination,  but  is  expensive 
to  construct. 

The  achromatic  condenser  of  Thomas  Grubb,  figured 
below  (Fig.  96),  in  which  A  is  a  piece  of  plain  glass 
to  act  as  a  protection  to  the  condensers ;  B  is  a 
plano-convex  sample  lens ;  c  a  plano-convex  achro- 
matised  ;  and  D  a  combination  very  much  over- corrected 


Fig.  96. 

for  colour,  and  of  slight  negative  power,  although  the 
externals  are  plane.  From  c  to  D  the  rays  are  nearly 
parallel.  Passing  through  D,  they  diverge  until  they  are 
received  by  a  large  lens  by  which  they  are  rendered 
convergent. 

Achromatic  Condensers. — Although  we  have  just  de- 
scribed two  achromatic  condensers,  we  do  not  consider 
that  these  are  necessary  for  lantern  work.  Chromatic 
aberration  is  reduced  to  some  extent  by  the  behaviour 
of  a  ray  which,  when  it  passes  through  the  first  lens 
(unduly  separated  from  the  second  for  the  sake  of 
illustration),  is  decomposed  as  shown  at  r  and  b  in  the 
figure  (97),  the  second  lens  having  a  tendency  to  bring 


WIDE-ANGLE  TRIPLE  CONDENSERS. 


249 


these  coloured  rays  together  again.  For  this  reason 
the  compound  condenser  was  formerly  designated  as 
achromatic. 


Fig.  97. 

Wide-Angle  Triple  Condensers- — When  lecturing  before 
the  Camera  Club  on  the  principles  which  underlay  the 
construction  of  a  condenser  that  would  transmit  a  larger 
amount  of  light  than  was  usual,  and  assuming  the  lime- 
light to  be  the  source  of  illumination,  we  inquired  the 
greatest  angle  of  light  possible  to  be  got  advantageously 
through  a  condensing  system,  as  this  lay  at  the  root  of 
the  whole  matter,  and  in  doing  so  had  to  ascertain  how 
near  can  the  light  be  approximated  to  the  first  surface 
with  safety.  From  innumerable  trials  with  lenses  of  a 
thickness  not  too  great,  and  set  with  such  a  degree  of 
looseness  in  the  brass-work  as  not  to  be  cell-bound,  we 
find  that  two  inches  may  be  considered  as  quite  safe. 
When  condensers  crack,  it  is  usually  the  result  of  their 
being  too  tightly  burnished  in  their  cells,  brought  too 
suddenly  under  the  influence  of  the  heat  of  the  radiant, 
or  being  subjected  to  currents  of  cold  air.  We  assume, 
of  course,  the  perfect  annealing  of  the  glass  of  which 
they  are  formed. 


250  FUNCTIONS  OF  THE  CONDENSER. 

Functions  of  the  Condenser. — At  this  stage  we  proceed 
to  analyse  the  functions  of  a  lantern  condenser,  so-called. 
We  find  that  these  are  (i)  the  collecting  and  (2)  the 
condensing  of  the  light.  Of  these,  the  former  is  much 
the  more  important.  What  we  wish  done  is  the  collec- 
tion of  so  many  rays  as  to  form  a  large  angle,  and  their 
projection  forward  in  as  near  an  approach  to  parallelism 
as  possible.  Absolute  parallelism  cannot  be  obtained 
unless  the  flame  were  a  point,  instead  of  being,  as  it  is,  a 
disc  or  patch  having  sensible  dimensions  of,  say,  a  quarter 
of  an  inch  upwards. 

Some  of  the  cheap  French  condensers  (of  which  we 
would  not  speak  disparagingly,  for  they  render  excellent 
service,  and  are  marvels  at  their  price)  transmit  an  angle 
of  light  of  from  40°  to  50°,  and  a  superior  class  of  London- 
made  articles  claims  to  embrace  60°.  But,  by  a  slightly 
increased  expenditure  of  optical  means,  it  is  possible  to 
increase  this  angle  to  95°,  which  somewhat  more  than 
doubles  the  intensity  of  the  illumination.  Let  us  see  in 
what  way  this  is  to  be  accomplished. 

Kepler's  law  is  that  the  focus  of  a  plano-convex  lens 
equals  the  diameter  of  the  sphere  of  convexity.  This  is, 
of  course,  for  parallel  rays,  and  it  is  those  we  are  dealing 
with  at  present ;  and  we  are  also  dealing  with  plano- 
convex lenses,  these  being  the  best  for  condensers,  sub- 
ject, perhaps,  to  a  slight  hollowing  of  the  flat  surface. 
Well,  it  is  very  evident  that,  if  we  desire  a  large  angle  of 
light,  the  single  Kepler  won't  do  much  for  us,  unless, 
indeed,  it  were  made  enormously  thick — even  hemi- 
spherical— when  we  would  encounter  two  evils.  First, 


COLLECTING  SYSTEM.  251 

the  enormous  spherical  aberration  consequent  upon 
transmitting  light  through  a  bull's-eye,  and,  secondly, 
the  proximity  of  the  said  bull's-eye  to  the  radiant,  which 
not  only  emits  light  but  heat — a  heat  which  would 
quickly  cause  our  bull's-eye  to  be  fractured.  How,  then, 
is  it  to  be  accomplished  ?  By  borrowing  the  ideas  of  the 
microscopist.  Who  ever  heard  of  a  microscopic  objective 
of  even  the  most  distant  pretensions  to  wide  angle  being 
composed  of  one  lens  ?  Well,  no  more  is  it  possible  in 
our  collecting  system,  which  is  analogous. 

Collecting  System. — We  must  have,  at  least,  two  lenses 
for  our  purpose.  One  of  them — that  nearest  to  the  light 
— must  be  4^  inches  in  diameter  in  order  to  catch  up  the 
95°  spoken  of.  But  this  cannot  render  the  rays  parallel ; 
still,  it  transmits  them  to  its  colleague  under  such  cir- 
cumstances that  it  does  so,  the  two  lenses  thus  doing 
what  no  one  singly  could  effect.  The  first  lens  of  the 
collecting  system  is  comparatively  thin,  which,  apart 
from  any  optical  advantage,  is  useful  in  this  respect,  that 
it  has  to  bear  the  first  impact  of  the  heat,  and  this  lessens 
the  liability  to  fracture.  It  is  only  sixteen  mm.  (f  inch) 
thick  in  the  centre,  is  eight  to  nine  inches  focus,  and  is 
formed  by  preference  of  flint  glass.  The  second  element 
is  five  inches  in  diameter,  and,  the  radius  of  curvature 
being  rather  shorter,  this,  combined  with  its  greater 
diameter,  causes  it  to  be  proportionally  thicker,  being 
twenty-eight  mm.  (i^g-  inch)  at  its  centre,  and  seven  inches 
focus.  This  lens,  too,  should  be  made  of  colourless  glass. 
The  loss  of  light  from  absorption  is  trivial,  and  that  from 
oblique  incidence  is  really  so  little  as  to  be  unworthy  of 


252  COLLECTING  SYSTEM. 

notice,  but  it  carries  with  it  its  compensation,  for  it  occurs 
most  at  the  thinnest  portions  of  the  lens,  where  there  is 
the  least  absorption,  and  thus  aids  in  ensuring  uniformity 
of  illumination  throughout  the  entire  beam.  But  it  may 
be  reduced  by  rendering  the  first  surface  concave  instead 
of  plane,  and  retaining  the  balance  of  power  by  grinding 
the  back  surface  on  a  tool  of  shorter  radius.  At  one 
time  we  were  much  in  love  with  the  meniscus  form  of 
lens  for  this  purpose,  but,  after  many  trials  with  lenses 
both  piano,  meniscus,  and  plano-convex,  and  formed  of 
different  kinds  of  glass,  from  St.  Gobain's  crown  to 
English  flint,  we  arrived  at  the  conclusion  that  the  plane 
surface  answered  every  purpose. 

If  the  radiant  were  infinitesimally  small,  a  parallel 
beam  of  a  large  collected  angle  could  be  transmitted 
with  a  singular  degree  of  perfection  for  several  yards. 
With  a  triple  collecting  system  (that  worke  1  out  by  Dr. 
Charles  Cresson,  in  which  the  first  lens  is  a  plano-convex 
4^  inches  radius,  the  second  a  meniscus,  respectively  30 
inches  and  6  inches ;  and  the  third  a  crossed  lens  of 
52  inches  and  8f  inches  radii)  we  projected  a  very  tiny 
gaslight  on  to  the  dial  of  a  French  clock  several  yards 
distant,  which  was  thus  illuminated  a  whole  season.  But 
such  extreme  nicety  is  not  required  in  the  practical 
working  of  the  optical  lantern,  as,  owing  to  the  magni- 
tude of  the  flame,  two  elements  answer  every  purpose. 
The  two  described  should  be  mounted  together  as  closely 
as  possible,  fixed  permanently  in  the  lantern,  and  must 
always  be  used  together,  and  not  separate.  Until  a 
compound  collecting  system  of  this  nature  is  tried,  one 


THE  LANTERN  OBJECTIVE.  ^53 

can  form  no  idea  of  the  capabilities  of  the  lantern  for 
certain  scientific  purposes,  such  as  polarising. 

Condensing  System. — We  now  direct  attention  to  the 
condensing  element  of  this  optical  system.  We  have  seen 
that  the  two  elements  of  the  collecting  portion  must  be 
fixed  and  inseparable.  This,  on  the  contrary,  should  be 
variable,  and  selected  to  suit  the  special  end  in  view. 
Its  form  may  be  plano-convex,  more  especially  if  for 
use  with  long-focus  objectives ;  but  if  the  latter  is  to  be 
short-focus,  and  the  condenser  of  crown  glass,  then  is 
the  crossed  form,  in  which  the  curves  are  as  one  to  six 
or  two  to  thirteen,  open  to  be  preferred. 

But  dealing,  as  we  now  are,  with  immergent  parallel 
rays,  it  were  folly  to  imagine  that  a  condenser  properly 
adapted  for  an  objective  of  12  inches  focus  will  answer 
equally  well  for  one  of  6  inches.  Bearing  in  mind 
Kepler's  law,  which,  however,  applies  only  to  ojie  kind 
of  glass,  and  must  not  be  held  as  applicable  equally  to 
the  flint  glasses,  especially  those  of  the  denser  sort  pro- 
curable at  the  present  day,  we  would  say  that  for  long- 
projection  lenses  of  12  to  1 5-inch  focus  a  plano-convex 
having  a  radius  of  curvature  of  7  inches  will  serve  every, 
purpose;  for  an  objective  of  8  to  10  inches  the  radius 
may  be  4^  inches,  while  for  one  of  6  to  8  inches  4  inches 
will  suffice.  But,  as  we  have  said,  this  latter  may  with 
advantage  be  a  crossed  lens,  in  which  case  the  radius  of 
the  more  convex  side  will  be  longer. 

The  Lantern  Objective. — The  requirements  of  the 
lantern  objective  are  that  it  shall  receive  and  transmit 
all  the  light  that  passes  through  the  condensers,  ancj 


254  THE  LANTERN  OBJECTIVE. 

that  it  shall  give  a  flat  field  with  good  definition 
throughout.  Its  diameter,  especially  that  of  its  posterior 
combination,  must  be  sufficiently  large  to  take  in  not 
merely  the  whole  of  the  cone  of  rays  emerging  from  the 
condenser,  but  by  preference  a  little  more.  This  permits 
of  the  utilisation  of  a  small  portion  of  light  radiated 
from  the  substance  of  the  image  itself. 

A  large  back  lens  also  permits  it  to  be  brought  nearer 
to  the  picture,  and  this  is  advantageous,  especially  with 
the  condensers  of  the  common  order,  as  it  acts  in  con- 
densing the  scattered  rays  from  those  of  this  class, 
enabling  also  the  light  to  be  approached  nearer  to  the 
condenser.  The  lens  tube  should  be  longer  than  in  the 
case  of  its  application  to  photography,  for,  unlike  this 
all  it  is  required  to  cover  is  the  very  limited  area  com-' 
prised  in  a  plate  three  and  a  quarter  inches  square,  minus 
the  portion  occupied  by  the  mat.  For  the  highest  class 
of  objective,  it  suffices  that  it  be  achromatic  in  the  sense 
different  from  actinic,  for,  so  long  as  the  visual  image  is 
perfect,  it  matters  not  what  becomes  of  the  violet  or 
chemical  rays,  or  what  relations  they  have  to  the 
luminous  ones. 

It  is  in  the  construction  of  a  lantern  objective  of 
short  focus  that  the  skill  of  the  optician  is  taxed,  as  it 
has  to  cover  sharply  to  the  margin  with  its  full  aperture, 
and  under  circumstances  in  which  the  slightest  inequality 
in  the  definition  is  instantly  detected.  To  a  cultivated 
eye  it  is  extremely  unpleasant  to  see  an  image  quite 
sharp  in  the  centre  of  the  disc,  and  falling  off  rapidly 
towards  the  margin,  or  by  racking  in  securing  marginal 


THE  LANTERN  OBJECTIVE.  255 

sharpness  at  the  expense  of  the  centre.  Of  the  various 
forms  of  objective  to  be  met  with,  at  any  rate  for  those 
of  medium  short  focus,  we  incline  to  give  preference  to 
that  introduced  ten  or  twelve  years  ago  by  J.  H.  Dall- 
meyer,  judging  by  a  comparison  of  the  performance  of 
one  of  this  class,  with  several  others  in  our  possession. 
In  it  the  mount  is  longer  and  the  elements  of  the  back 
lens  (see  Fig.  39,  page  78)  are  separated  to  an  extent 
which  would  prove  fatal  to  sharpness  in  the  case  of 
one  employed  in  producing  a  photographic  image  in  the 
camera.  If  photographic  lenses  are  to  be  employed  in 
the  lantern,  those  of  the  carte-de-visite  (Petzval  form), 
that  is,  those  corrected  for  flatness  of  field,  even  to  the 
extent  of  there  being  slight  astigmatism,  are  advan- 
tageous. One  of  the  most  satisfactory  short- focus  objec- 
tives we  ever  used  had  a  back  lens  two  and  a  quarter 
inches  in  diameter,  the  front  lens  being  one  and  three- 
quarter  inches.  We  gave  a  very  great  excess  of  negative 
spherical  aberration  to  the  back  lens,  and  the  front  was 
a  nearly  plano-convex  achromatic  of  short  focus.  This 
gave  a  field  which  was  singularly  flat,  the  definition  at 
the  margin  quite  equalling  that  in  the  centre ;  but, 
owing  to  the  excess  of  aberration  spoken  of,  the  image 
did  not  quite  equal  in  sharpness  that  obtained  by  the 
ordinary  carte-de-visite  lens  with  rounder  field.  Still, 
spectators  seated  at  a  distance  of  five  yards  from  the 
screen  were  unable  readily  to  appreciate  that  the 
definition  was  imperfect,  for,  as  is  well  known,  even  the 
crude  brushwork  of  the  scene-painter  seems  sharp 
when  viewed  from  a  distance. 


256  THE  LANTERN  POLARISCOPE. 

For  objectives  of  long  focus  there  does  not  appear  to 
be  the  same  tax  on  the  skill  of  the  optician.  Poor, 
indeed,  must  be  the  lens  of  ten,  twelve,  or  fourteen 
inches  focus  that  will  not  cover  sharply  and  uniformly  a 
plate  three  inches  in  dimensions. 

In  the  foregoing,  double  combinations  of  lenses  are 
implied,  but  single  achromatic  lenses  ,of  plano-convex 
or  slightly  meniscus  form  also  answer  as  objectives. 
Whether  they  are  used  singly  or  two  placed  close 
together,  their  convex  sides  must  be  next  the  slide,  and 
a  diaphragm  must  be  placed  outside. 

The  Lantern  Polariscope- — With  respect  to  polarisation 
of  light  by  the  lantern,  the  method  which  we  described 
at  the  Nottingham  meeting  of  the  British  Association 
fulfils  the  requirements  of  giving,  with  the  usual  lantern, 
a  much  more  intense  volume  of  polarised  light  than  is 
otherwise  obtainable. 

Without  going  into  too  great  detail,  it  may  suffice  to 
say  that  when  the  cone  of  light  from  the  condenser  is 
made  to  fall  upon  the  bundle  of  glass  plates  by  which  it 
is  polarised,  only  a  portion  of  the  light  is  thus  affected, 
for  as  the  angle  of  polarisation  is  an  exact  one,  none 
but  the  axial  rays  are  polarised  in  a  perfect  manner,  all 
the  others  impinging  upon  the  plates  at  other  than  the 
polarising  angle.  The  expedient  we  adopt  is  to  receive 
the  cone  upon  a  concave  lens,  by  which  the  cone  is  trans- 
formed into  a  cylindrical  bundle  of  rays,  every  one  of 
which  becomes  amenable  to  the  polarising  influence  of  the 
plates,  and  after  undergoing  the  change  they  are  brought 
into  a  state  of  convergence  by  means  of  a,  convex  lens, 


THE  LANTERN  POLARlSCOPE.  257 

This  system  applies  equally  to  the  analyser  or  Nicol 
prism,  as  to  the  polariser ;  in  either  case  a  considerable 
gain  in  the  light  accrues. 

In  certain  scientific  institutions  in  America,  where 
lantern  condensers  have  some  pretensions  to  be  called 
perfect,  the  polarising  of  a  large  volume  of  light  is 
effected  in  a  simple  and  most  excellent  manner.  By 
means  of  the  two  collecting  lenses  previously  described, 
the  light  from  the  lime  is  reduced  to  a  large  parallel 
beam,  which  falls  upon  a  bundle  of  glass  plates  placed 
at  the  usual  polarising  angle  of  56°,  and  after  reflection 
is  received  by  another  lens  of  the  same  diameter,  by 
which  it  is  condensed.  This  lens  is  of  either  long  or 
short  focus,  to  suit  its  special  requirement. 

Such  a  world  of  wonder  and  beauty  is  opened  up  by 
the  polarising  attachment  to  the  lantern,  that  it  is 
matter  of  surprise  it  is  not  more  common  than  it  is. 
To  polarise  the  light,  all  that  is  necessary  is  to  take  a 
packet  of  eight  or  ten  clean  and  rather  thin  quarter-plate 
size  glass  plates  of  the  best  quality,  and  having  bound 
them  all  tightly  together  by  the  edges,  place  them  in 
front  of  the  condenser,  at  a  little  distance  from  it,  and  at 
an  angle  of  fifty-six  degrees.  With  the  light  reflected 
from  this  parcel  of  plates,  all  the  phenomena  incident  to 
noiarised  light  may  be  obtained. 


CHAPTER  XXXVII. 

PHOTO-TELESCOPIC  LENSES. 

BY  telescopic  effects  is  here  understood  the  produc- 
tion of  an  image  necessitating,  under  ordinary  conditions, 
either  a  telescope  of  moderate  dimensions  having  its 
systems  of  eye-pieces  to  magnify  the  aerial  image,  or  an 
objective  of  unusually  long  focus.  What  is  required  is 
a  combination  that  magnifies  in  itself  while  permitting 
the  employment  of  a  camera  of  no  unusual  length. 

A  telescope  of  the  ordinary  kind,  having  its  eye- 
piece in  sitti,  gives  an  image  the  size  of  which  depends 
upon  the  dimensions  of  the  telescope  and  the  distance 
of  the  ground  glass  from  the  eye-piece.  This  image, 
however,  is  only  sharp  visually,  and  the  adjustment  for 
photography  necessitates  a  number  of  trials  in  order  to 
ascertain  the  position  of  the  chemical  focus.  Dr.  Dick, 
in  his  Practical  Astronomer  (1845),  describes  how  the 
telescope  may  be  used  for  throwing  an  image  of  the  sun 
up  to  thirty  inches  in  diameter  upon  a  screen  in  a 
camera  obscura  consisting  of  the  room  in  which  the 
spectators  are  seated.  This  was  for  the  purpose  of 
exhibiting  the  solar  spots  to  a  number  of  persons  at  a 
time. 

In  1870  we  published  a  simple  way  of  obtaining  a 
sharp  telescopic  view  of  the  sun  or  other  distant  object. 


TELEO-PHOTO  OBJECTIVES.  259 

An  aerial  image  is  formed  by  a  lens  corrected  for 
photography,  and  this  is  magnified  by  a  similar  lens  of 
short  focus  placed  the  requisite  distance  in  front  of  the 
aerial  image.  It  is  simple  and  answers  well. 

In  the  chapter  on  the  orthoscopic  lens  (page  60), 
we  have  spoken  of  the  property  possessed  by  it  of 
giving  a  larger  image  in  a  given  extent  of  camera  than 
that  obtainable  by  any  other  objective,  and  it  is  also 
known  to  many  that  a  greatly  enlarged  view  of  a  scene 
can  be  obtained  by  employing  an  ordinary  opera  glass 
as  the  objective,  the  large  lens  to  the  outside  of  course. 
We  long  ago  used  one  of  the  barrels  of  a  '  twelve-lens ' 
opera  glass,  that  is,  one  in  which  each  lens  was  achro- 
matised  by  being  formed  of  three  elements ;  but  felt 
dissatisfied  on  account  of  the  very  small  field  covered. 
What  was  covered,  however,  showed  an  image  of  greatly 
enlarged  dimensions. 

Dallmeyer's  Teleo-Photo  Objective. —  It  is  gratifying  to 
find  that  the  optician  named  has  been  directing  his 
attention  to  the  Galilean  method  of  forming  an  image, 
so  as  to  adapt  it  for  photographic  purposes.  The  image 
by  the  outer  or  object-glass,  which  may  be  either  a 
plano-convex  or  a  crossed  achromatic,  is,  previous  to 
arriving  at  its  focus,  intercepted  by  an  actinically  cor- 
rected negative  lens  of  greater  negative  power  than  the 
positive  power  of  the  other.  This  negative  lens  is  formed 
of  two  or  three  elements,  but  the  field  capable  of  being 
sharply  covered  is  limited.  The  degree  of  enlargement 
obtainable  is  determined  by  the  separation  of  the  lenses, 
coupled  with  the  distance  of  the  focussing  screens. 


CHAPTER  XXXViil. 

EXCEPTIONAL  RAPIDITY  WITH  HIGH  DEFINITION. 

Piazzi  Smyth's  Corrector. — We  have  previously  pointed 
out  that  when  a  portrait  combination  is  corrected  for 
flatness  of  field,  this  is  attainable  at  the  expense  of 
marginal  astigmatism,  unless  the  field  to  be  covered  be 
very  narrow.  One  on  the  contrary  that  is  corrected  to 
give  the  best  definition  at  the  margin  does  so  at  the 
expense  of  roundness  of  field,  so  that  when  the  centre  is 
in  focus  the  ^ides  are  out,  and  vice  versa. 

When  <C.  Piazzi  Smyth  was  Astronomer  Royal  for 
Scotland,  he,  knowing  well  the  highest  requirements  of 
a  photographic  lens,  devised  an  ingenious  means  by 
which  the  oblique  pencils  of  a  round  field  lens  could  be 
so  lengthened  as  to  eventually  render  the  whole  field 
flat. 

The  corrector  employed  for  this  purpose  consists  o* 
a  rather  thick  plate  of  glass  the  size  of  the  "sensitive 
plate,  one  side,  that  towards  the  front,  being  ground  to 
a  hollow  or  concave  curve,  the  other  side  being  flat. 
This  must  be  mounted  in  front  of  and  as  close  to  the 
sensitive  plate  as  possible. 

The  action  of  the  corrector  is  as  follows  : — The  axial 
rays  from  the  lens  fall  upon  its  centre,  where  it  is  very 


PIAZZI  SMYTfTS  CORRECTOR.  261 

thin,  and  although  the  convergence  is  affected,  it  is  only 
so  in  a  slight  degree,  and  the  rays  come  to  a  focus  upon 
the  plane  of  representation.  But  at  the  margin  the  rays 
have  to  pass  through  a  considerable  body  of  glass,  which 
they  do  in  a  degree  more  nearly  approaching  parallelism 
than  previous  to  their  entrance,  and  upon  emerging  from 
the  flat  surface  they  have  still  to  travel  a  little  farther 
before  being  brought  to  a  focus  on  the  sensitive  plate. 

As  the  curved  surface  of  the  corrector  stands  so 
nearly  normal  to  the  rays  there  is  scarcely  any  loss 
from  oblique  incidence,  while  there  is  a  very  decided 
gain  in  rapidity  in  consequence  of  the  large  aperture 
that  can  be  given  to  the  lens. 


CHAPTER  XXXTX. 

MISCELLANEOUS. 

To  Remove  Lacquer  from  Mounts.  —  By  immersing  the 
brass  work  in  boiling  water  in  which  washing  soda  or 
potash  is  dissolved,  the  lacquer  will  be  immediately 
removed. 

A  better  way,  and  one  by  which  the  necessity  for 
heating  is  obviated,  consists  in  applying,  by  means  of  a 
tuft  of  cotton  wool,  a  mixture  of  equal  parts  of  alcohol 
and  ammonia. 

Lacquering.  —  Ordinary  lacquering  necessitates  the 
heating  of  the  mount  in  order  to  its  close  adhesion,  and 
not  drying  with  a  chilled  surface.  But  if  the  lacquer  be 
rendered  alkaline  by  the  addition  pf  a  small  proportion 
of  ammonia,  it  will  dry  bright  without  heat.  This 
applies  in  a  special  manner  to  lacquers  of  which  shellac 
forms  the  main  constituent. 

An  excellent  tough  transparent  coating  for  brass  is 
obtained  by  coating  it  cold  with  a  solution  of  celluloid 
in  acetate  of  amyl  (or  acetone).  It  takes  several  hours 
to  become  quite  dry,  but  is  then  hard  and  durable. 

Staining  Brass  Black. — This  system  consists  in  stain- 
ing the  surface  of  the  metal  in  contradistinction  to 
applying  an  opaque  black  varnish. 

A  black  which  penetrates  the  surface  well,  consists 


STAINING  BRASS  VARIOUS  COLOURS.          263 

in  immersing  the  article,  previously  made  clean  and 
freed  from  greasiness,  in  a  weak  solution  of  a  mixture  of 
the  nitrates  of  copper  and  silver,  and  then  exposing  to 
heat  till  the  colour  was  well  developed,  afterwards 
plunging  into  water.  In  this  mixture  the  copper  should 
largely  predominate. 

A  method  of  staining,  without  applying  heat,  con- 
sists in  suspending  the  article  for  a  short  time  in  a 
solution  composed  of  one  ounce  of  carbonate  of  copper, 
dissolved  in  eight  ounces  of  ammonia,  to  which  is 
then  added  sixteen  ounces  of  water.  The  carbonate  of 
copper  is  obtained  by  dissolving  sulphate  of  copper  in 
water,  and  carbonate  of  potash  in  another  quantity  of 
water,  and  pouring  one  into  the  other.  Decomposition 
immediately  takes  place,  the  carbonate  of  copper  being 
precipated.  Pour  off  the  supernatant  liquid  and  wash 
in  two  or  three  changes  of  water. 

Staining  Brass  various  Colours. — Although  the  author 
cannot  conceive  of  lens  fittings  being  stained  other  than 
black,  yet  such  a  variety  of  really  beautiful  colours  was 
obtained  by  the  following  process  before  the  brass  took 
on  a  black  stain,  that  it  may  be  well  to  record  it.  Every 
photographer  knows  that  a  solution  of  hyposulphite  of 
soda  is  immediately  decomposed  by  the  addition  of  a 
variety  of  salts  and  by  acids.  Dissolve  three-quarters 
of  an  ounce  of  hyposulphite  in  a  half-pint  of  water,  and 
in  another  half-pint  of  water  dissolve  three-quarters  of 
an  ounce  of  acetate  of  lead.  Mix  the  two  solutions  and 
warm  them,  then  at  once  immerse  the  articles  to  be 
coloured, 


264  DEAD  BLACK  VARNISH. 

Instead  of  the  acetate  of  lead  we  have  employed 
sulphuric  acid  in  very  small  quantity  to  effect  the  de- 
composition of  the  hyposulphite.  Scarcely  a  colour  can 
be  named  which  the  brass  (which  must  be  scrupulously 
clean)  will  not  assume  in  successive  stages  of  the 
immersion. 

Some  stain  brass  to  a  good  black  colour  by  brushing 
it  with  a  dilute  solution  of  nitrate  of  mercury,  followed 
by  two  or  more  applications  of  a  solution  of  sulphide  of 
potassium  (liver  of  sulphur). 

Dead  Black  Varnish. — This  may  be  made  in  several 
ways,  among  others  by  stirring  lamp  black  intimately 
with  a  rather  thin  Brunswick  black,  the  quantity  of  the 
former  being  such  as  to  ensure  its  drying  with  a  dead 
surface. 

The  best  opticians  employ  a  mixture  of  vegetable 
black  and  lacquer,  the  proportions  being  determined  by 
trial.  The  quantity  of  black  must  be  just  such  as  to 
cause  it  to  dry  dead  and  no  more.  A  good  way  of 
mixing  them  is  to  place  an  ounce  of  ordinary  gun-shot 
with  it  in  the  bottle,  and  shake  well  up.  The  article 
must  be  heated  ere  this  varnish  is  applied. 

One  of  the  toughest  dead  black  varnishes  we  have 
ever  tried  is  obtained  from  importers  of  American 
lacquers.  It  is  sold  under  the  name  of  enameloid, 
and,  so  far  as  we  can  see,  is  composed  of  celluloid 
dissolved  in  acetone,  with  the  requisite  quantity  of 
vegetable  or  other  black  added  to  ensure  deadness.  It 
is  applied  cold,  and  dries  in  one  or  two  hours. 

I 'ocussirg  Screen  for  Lens  Testing. — If  an   extremely 


FOCUSSING  SCREEN.  265 

fine  grey  glass  surface  be  required  for  receiving  a  small 
and  delicate  image,  as  in  testing  lenses  or  for  photo- 
microscopic  focussing,  the  ground  glass  obtained  in 
commerce  may  not  unfrequently  be  found  to  be  too 
coarse.  A  very  fine  grain  can  be  obtained  by  exposing 
a  sheet  of  scrupulously  cleaned  patent  plate  to  the  fumes 
of  fluoric  acid  generated  in  the  following  manner : — 
Having  obtained  a  moderately  deep  vessel  formed  of 
sheet  lead,  gutta  percha,  or  vulcanite,  sprinkle  the 
bottom  with  some  finely  crushed  fluor  spar,  and  over 
this  pour  a  little  sulphuric  acid.  Acid  fumes  will  be 
immediately  generated,  and  by  allowing  them  to  act 
upon  the  surface  of  the  glass  this  becomes  corroded,  the 
grain  at  first  being  exceedingly  fine  and  yet  capable  of 
arresting  an  image  thrown  upon  it  by  a  lens. 


INDEX. 


Aberration— What  is  it  ?  6 
Abbe,  Professor,  177 
Abbe's  lens,  183 
Achromatic  condensers,  248 
Achromatism,  Testing  for,  137 
Achromats,  New,  179 
Old,  179 
Actinism,  3 
Adjusting  dissimilar  lenses,  154 

,,          uncorrected  lenses,  29 
Advantage  of  large  diaphragm,  131 

,,  single  lenses,  48 

Aerial  images,  171 
Albo-carbon  light,  243 
Aldis,  H.  L.,  201,  203 

„      Lens,  204 
Aluminium  mounts,  211 
Angle  of  view  included  by  lens,  165 

,,  measuring,  166 

Angular  aperture,  75 
Antiplanet,  Dr.  A.  Steinheil's,  72, 

181,  192,  203 

Aperture  in  diaphragm,  147 
Aplanat,  Dr.  A.  Steinheil's,  177 
Aplanatism,  Meaning  of,  129 
Arbeit,  E.,  200 

Arborescent  marks  in  lenses,  226 
Architecture,  Lens  for,  236 
Aristostigmat,  Dr.    Hugo  Meyer's, 

199 
Ascertaining  angle  of  view,  165 

,,  equivalent  focus,  101 

Astigmatism,  How  to  test  for,  139, 

177 

Astigmatism,  to  correct,  179 
Axial  versus  oblique  rays,  3 

Back  combinations,  Properties  of,  76 


Balsaming  lenses,  229 

Barrel  distortion,  52 

Bayonet  joints  for  lens  fittings,  207 

Beck  on  aberration,  18 

Blackening  brass,  262 

Blocking  lenses,  222 

Bow's    compensating     method    for 

single  lenses,  29 
Bow's  method  for  equalising  light, 

153 

Brewster's  graduated  eye  tube,  123 
Busch  Anastigmat,   Karl   Martin's, 

199,  208 
Butterfly  stop  for  equalising  light, 

153 

Camera  Club  focimeter,  119 
,,       for  lens  testing,  135 

Canada  balsam,  159 

Casket  lenses,  88 

Cause  of  discolouration  of  glass,  161 
,,        distortion,  51 

Cell-bound  lenses,  21 1 

Cementing  lenses,  224 

Central  rays,  Aberration  of,  15 

Chemical  and  visual  foci,  2 

Chromatic  aberration,  9 

Circular  diaphragms,   Reasons   for, 
148 

Clark,  Alvan,  191 

Claudet  on  conjugate  foci,  107 

Collecting  system,  251 

Collinear  lens,  187 

Coloured  light,  its  properties,  4 

Colouring  glass,  Theories  of,  162 

Colourless  glass,  158 

Combination  landscape  lens,  62 

Combining  lenses,  Rule  for,  157 


INDEX. 


267 


Compound  lenses,  Flare  in,  96 
Concave  lens,  Effect  produced  by, 

209 

Concentric  lens,  182 
Condenser,  its  uses,  219 
Conjugate  foci,  106 

,,         focus,  formulae  for,  114 
Cooke  lens,  201,  205 
Copying  maps,  Lens  for,  236 

,,       portraits,  235 
Covering  power,  136 
Cresson's  condenser,  252 
Crossed  lens,  Centre  of,  36 
'  Crown,'  178 
Cundell's  lens,  54 
Curing  distortion,  54 

,,      existing  distortion,  237 

,,      over-correction,  144 
Curvature  of  straight  lines,  50 

Dallmeyer's  diffusion  objective,  130 
,,  non  -  distorting    land- 

scape lens,  47 

Dallmeyer's  portrait  combination,  78 
, ,          triple  achromatic,  63 
,,     .     wide-angle    rectilinear, 

71 

Davidson's  combination,  70 
Dead  black  varnish,  264 
Deep  meniscus  lenses,  27 

The,  31 

Defective  cementing,  228 
Dense  flint  glass,  Deterioration  of, 

1 60 

Dense  glass,  Advantage  of,  82 
Density  influences  curvatures,  216 
Depth  increased  by  small  aperture, 

125 

Depth  of  focus,  123 

,,      produced  by  a  stop,  23 
Diaphragm,  Advantage  of  large,  131 

,,          Apertures  in,  147 
Diaphragms  standing  at  angle,  151 

,,  their  functions,  20 

Diffusion  by  single  lenses,  134 
,,        of  focus,  128 
„  ,,     Claudet's  method, 

133 


Discolouration  of  cement,  227 
Dissimilar  lenses,  Adjustment  of,  154 
Distortion,  49 

„         of  convergence,  49,  238 
, ,         of  curvature,  its  cure,  237 
„         of  curvature   and    con- 
vergence combined,  240 
Double  condensers,  245 

,,  best  form,  247 

Doublets  of  Grubb  and  Ross,  68 

„          single  lenses,  31 
Dynar,  Dr.  Har ting's,  198 

Edging  and  centering,  222 

Elasticity  of  focus,  32 

Elements  of  combination  as  land- 
scape lenses,  91 

Emery  preparing  for  lens  grinding, 
220 

Enameloid,  264 

Enlarging  and  reducing,  113 
,,         landscapes,  231 

Equalising  theskiesand  foregrounds, 
150 

Equi-double  convex,  Rule  for  focus, 
216 

Equivalent  focus,  100 

, ,  Grubb's    method 

of  ascertaining,  102 

Euryplan,  E.  Arbeit's,  200 

Fashion  in  lenses,  56 

First  diffusion  of  focus  lens,  130 

Fitting  focussing  telescope  to  camera, 

172 

Fixed  focus  for  landscapes,  126 
Flange  apertures,  212 
Flare  in  rectilinears,  97 

„     spot,  93 

,,         ,,    How  to  discover,  142 
Flatness  of  Field,  Testing  for,  140 
'  Flint,'  178 
Focal  centre  of  a  combination,  38 

,,     range,  Table  showing,  127 
Focus  adjuster,  89 

„      Diffusion  of,  128 
Focussing  by  telescope,  170 
„        screens,  170,  265 


INDEX. 


Focussing  with  working  stop,  26 
Form  of  lens  for  enlarging,  230 
Functions  of  a  condenser,  250 
Fuzzy  pictures,  Means  for  producing, 
133 

Gauss  correction,  191,  199 
Genesis  of  doublet,  69 
Glass  affected  by  light,  161 

,,     selecting  by  polariscope,  214 
Globe  lens,  66 
Goddard's  double  periscopic,  61 

,,         triple  lens,  62 
Goerz  Double  Anastigmat,  I  ,  195 
Goerz    Double     Anastigmat,     HA. 

(Convertible),  189 
Goerz  Double  Anastigmat,  III.,  186 
„  ,,     Hypergon,  194 

Grinding  curved  margins  of  lenses,  4  5 

,,       lenses,  220 
Groups,  Lenses  for,  235 
Grubb,   Sir  Howard,   on  conjugate 

foci,  116 

Grubb's  aplanatic,  44 
,,       condensers,  248 
,,       T.,    method    for   conjugate 
foci,  109 
Grubb,  T.,  184 

Hand  cameras  and  conjugate  foci, 

112 

Harting,  Dr.,  196 
Helivar,  Dr.  Harting's,  196 
Hoegh,  Emil  von,  186,  189,  205 
Holostigmat,  Watson  &  Sons',  205 
How  to  find  focal  centre,  39 
Hypergon  Double  Anastigmat,  194 

Ideal  definition,  7 
Images  in  telescope,  171 
Imperfect  mounting  causes  flare,  93 
Inverted  image,  Cause  of,  12 
Iris  diaphragm,  148 

Jena  glasses,  178,  205 

Kampfer,  Dr.,  187 
Kepler's  law,  250 


Lacquering,  262 

Lacquer,  to  remove  from  mounts,  262 
Landscape  lenses,  Flare  in,  96 
„          Lenses  for,  235 
,,          lens,  Mounting,  46 
Lantern  objectives,  253 
,,       optics,  241 
,,        Polariscope,  256 
Large  micro  objects,  236 

,,      portrait  lenses  without  depth, 

25 

Laws  governing  conjugate  foci,  107 
Leitz,  Ernst,  200 
Lens  grinding,  214 

,,  tools,  217 

Lenses,  Forms  of,  10 

,,       interchanging  their  element, 

33 
Lenses  of  unequal  curvature,  Foci  of, 

217 
Light,  action  of,  on  Canada  balsam, 

162 
Light,  action  of,  on  glass,  160 

,,      causes  deterioration  of  lenses, 

.158 
Light,  Decomposition  of,  3 

,,      for  enlarging  or   projecting, 

241 
Limelight,  242 

Manganese  in  glass,  162 

Marcey  lamp,  242 

Martin,  Karl,  199,  205 

Matching  lenses,  154 

Mechanical  means  of  estimating  con- 
jugate foci,  117 

Mechanical  not  the  focal  centre,  40 

Mineral  oil  lamps,  241 

Misconceptions    regarding    dia- 
phragms, 22 

Morrison's  rapid  doublet,  84 
,,          wide  angle,  67 

Mounts  and  cells,  207 

Nature  of  focussing  telescope,  172 
Negative  aberration,  Lenses  giving, 

17 
Non-achromatic  lenses,  28 


INDEX. 


269 


Noton's  diaphragm,  148 


Object  glasses    for    focussing    tele- 
scope, 173 
Oblique  diaphragms,  150 

,,       rays,  Aberration  of,  16 
Opaque  stop,  Equalising  by,  152 
Optical  centre  of  single  lens,  34 

,,      centre,  Properties  of,  37 

,,      contact,  199 

,,      flare  spot,  94 

,,      perfection    not     necessary, 

131 

Orthoscopic  lens,  55 
Orthostigmats,   Dr.  R.  Steinheil's, 

187,  195 
Over  and  under  correction,  138 

Panoramic  lens,  65 

Pebble  lenses,  6 

Periplan,  Ernst  Leit/.'s,  200 

Petzval  lenses  for  lantern,  255 

Petzval's  portrait  combination,  76, 
177 

Photographic  correction  a  compro- 
mise, 16 

Photographic  definition,  8 

Piazzi  Smyth's  corrector,  260 

Pincushion  distortion,  53 

Pinhole  aperture  for  testing  equiva- 
lent focus,  103 

Pinhole  apertures,  13 

Planar  Lens,   Dr.  Rudolph's,  191, 
206 

Plano-convex  lens,  Rule  for  focus, 
216 

Polariscope  for  lantern,  256 

Polishing  lens,  221 

Portrait  lenses,  History  of,  73 

Portraiture,  Lenses  for,  234 

Positive  and  negative  aberration,  17 

Protar  Lenses,  Dr.  Rudolph's,  184 
,,        VII.,  188 
„        VIlA.  (Convertible), 
1 88 

Protractors,  Use  of,  168 

Purity  of  glass,  142 

Putty  powder,  222 


Quality  of  image  by  altering  lenses, 
156 

Rapid  lenses,  Nature  of,  80 

,,      rectilinears,  81 
Rathenower  Optische  Industrie  An- 

stalt,  200 

Razor-edge  definition,  132 
Rectilinearity,  Testing  for,  143 
Rectilinear  landscape  lenses,  46 
Refraction  influenced  by  density  of 

glass,  5 

Remedy  for  flare,  94 
Removing  lacquer,  262 
Rohr,  Dr.  M.  von,  180 
Ross,  T.,  184 

Rudolph,  Dr.  P.,  179,  184,  185,  206 
Rudolph's  wide-angle  anastigmats, 

185 
Rule  for  estimating  foci,  in 

,,        focus  of  equi-double  convex, 

216 

Rule  for  focus  of  plano-convex,  216 
„     of  three  applied  to  focus,  104 

Schott,  Dr.,  178 
Schroeder,  Dr.  H.,  182,  183 
Schroeder  and  Stuart's  lens,  182 
Schulze  Gebriider,  201 
Secondary  image,  Cause  of,  95 
Selection  of  lenses,  230 
Separating  lenses,  Effect  of,  155 
Separation  of  lenses  not  arbitrary, 

209 

Shanks,  218 
Sharpness  conferred  by  diaphragms, 

23 

Single  lenses  compensated,  27 
,,      achromatic  lenses,  42 

Size  of  image,  155 

,,    image  determined  by  focus,  13 

Slit  apertures  in  diaphragms,  149 

Slowness  of  lenses,  Causes  of,  159 
,,  wide-angle  lenses,  166 

Specimen  lens  curves,  223 

Spherical  aberration,  15,  179 

Staining  brass  black,  262 

,,  various  colours,  263 


270 


INDEX. 


Steinheil,  Dr.  A.,  181,  192,  203 

Dr.  R.,  179,  186 
Steinheil's  periskop,  68 

,,         wide-angle  aplanat,  71 
Stereoscopic  lenses,  154,  233 
Stigmatic  Lens,  203 
Stop,  position  of,  in  single  lenses, 

210 

Stop  reducing,  and  its  object,  125 
Strise  in  lenses,  How  to  find,  141 
Summar,  Ernst  Leitz's,  200 
Surface  finish  of  lenses,  141 
Sutton's  triplet,  63 
Symmetry,  83 

Taylor,  Harold  Dennis,  201,  206 
Teleo-photo.  objectives,  258 
Telescope  for  focussing,  170 

,,         with   two  object-glasses, 

176 
Telescopic  definition,  8 

,,         effects    without    a    tele- 
scope, 258 
Templates,  218 

Tessar  Lens,  Dr.  Rudolph's,  193 
Testing  focus  by  single  glass,  104 

,,       for  aplanatism,  145 

,,        ,,  definition,  142 

,,       lenses,  135 

,,       points  to  be  noted,  136 


Treatment  of  lenses  for   solar   en- 
largements, 164 

Triple  condensers,  247 
„      lens,  63 

Unar  Lens,  Dr.  Rudolph's,  192 
Unequal  illumination,  Cause  of,  60 
,,        illumination   of  negatives, 

IS2 

Universal  flange  adapters,  213 
,,         landscape  lenses,  87 
,,         lens  on  new  system,  88 
Unofocal  Lens,  Dr.  R.  Steinheil's, 
I95»  205 

Voigtlander's  orthoscopic  lens,  58, 
187 

Waterhouse  diaphragms,  79 

Watson  £  Sons,  205 

What  constitutes  photographic  op- 
tics, i 

Whimsical  diaphragms,  148 

Wide-angle  lenses,  65 

,,          lenses  for  narrow  views, 
1 66 

Wide-angle  triple  condensers,  249 

Woodman's  table  of  view  angles,  1 15 

Zeiss'  lenses,  183 
Zentmayer's  lens,  68 


London:  STBANGEWAYS  &  Soys, Printers,  Tower  Street,  Cambridge  Circus,  W.C> 


HAVE    ONLY 

THREE  SIMPLE  GLASSES 

and  as  these  are  adjustable,  the  makers  are  able  to 

eliminate   astigmatism,    curvature    of  field,    spherical 

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of  lenses. 


The  late  J.  TRAILL  TAYLOR  said  :- 

'  Englishmen   are  justified  in  feeling  proud  of  a    lens 
of  the  excellence  and  undoubted  originality  of  this  one! 


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